diff --git a/src/api/endpoints/users/followers.js b/src/api/endpoints/users/followers.ts
similarity index 72%
rename from src/api/endpoints/users/followers.js
rename to src/api/endpoints/users/followers.ts
index 598c3b6bcf..011a1c70ce 100644
--- a/src/api/endpoints/users/followers.js
+++ b/src/api/endpoints/users/followers.ts
@@ -3,7 +3,7 @@
 /**
  * Module dependencies
  */
-import * as mongo from 'mongodb';
+import it from '../../it';
 import User from '../../models/user';
 import Following from '../../models/following';
 import serialize from '../../serializers/user';
@@ -20,33 +20,24 @@ module.exports = (params, me) =>
 	new Promise(async (res, rej) =>
 {
 	// Get 'user_id' parameter
-	const userId = params.user_id;
-	if (userId === undefined || userId === null) {
-		return rej('user_id is required');
-	}
+	const [userId, userIdErr] = it(params.user_id, 'id', true);
+	if (userIdErr) return rej('invalid user_id param');
 
 	// Get 'iknow' parameter
-	const iknow = params.iknow;
+	const [iknow, iknowErr] = it(params.iknow).expect.boolean().default(false).qed();
+	if (iknowErr) return rej('invalid iknow param');
 
 	// Get 'limit' parameter
-	let limit = params.limit;
-	if (limit !== undefined && limit !== null) {
-		limit = parseInt(limit, 10);
-
-		// From 1 to 100
-		if (!(1 <= limit && limit <= 100)) {
-			return rej('invalid limit range');
-		}
-	} else {
-		limit = 10;
-	}
+	const [limit, limitErr] = it(params.limit).expect.number().range(1, 100).default(10).qed();
+	if (limitErr) return rej('invalid limit param');
 
 	// Get 'cursor' parameter
-	const cursor = params.cursor || null;
+	const [cursor, cursorErr] = it(params.cursor).expect.id().default(null).qed();
+	if (cursorErr) return rej('invalid cursor param');
 
 	// Lookup user
 	const user = await User.findOne({
-		_id: new mongo.ObjectID(userId)
+		_id: userId
 	}, {
 		fields: {
 			_id: true
@@ -61,7 +52,7 @@ module.exports = (params, me) =>
 	const query = {
 		followee_id: user._id,
 		deleted_at: { $exists: false }
-	};
+	} as any;
 
 	// ログインしていてかつ iknow フラグがあるとき
 	if (me && iknow) {
@@ -76,7 +67,7 @@ module.exports = (params, me) =>
 	// カーソルが指定されている場合
 	if (cursor) {
 		query._id = {
-			$lt: new mongo.ObjectID(cursor)
+			$lt: cursor
 		};
 	}
 
diff --git a/src/api/endpoints/users/following.js b/src/api/endpoints/users/following.ts
similarity index 72%
rename from src/api/endpoints/users/following.js
rename to src/api/endpoints/users/following.ts
index 36868d6d5c..df5c058354 100644
--- a/src/api/endpoints/users/following.js
+++ b/src/api/endpoints/users/following.ts
@@ -3,7 +3,7 @@
 /**
  * Module dependencies
  */
-import * as mongo from 'mongodb';
+import it from '../../it';
 import User from '../../models/user';
 import Following from '../../models/following';
 import serialize from '../../serializers/user';
@@ -20,33 +20,24 @@ module.exports = (params, me) =>
 	new Promise(async (res, rej) =>
 {
 	// Get 'user_id' parameter
-	const userId = params.user_id;
-	if (userId === undefined || userId === null) {
-		return rej('user_id is required');
-	}
+	const [userId, userIdErr] = it(params.user_id, 'id', true);
+	if (userIdErr) return rej('invalid user_id param');
 
 	// Get 'iknow' parameter
-	const iknow = params.iknow;
+	const [iknow, iknowErr] = it(params.iknow).expect.boolean().default(false).qed();
+	if (iknowErr) return rej('invalid iknow param');
 
 	// Get 'limit' parameter
-	let limit = params.limit;
-	if (limit !== undefined && limit !== null) {
-		limit = parseInt(limit, 10);
-
-		// From 1 to 100
-		if (!(1 <= limit && limit <= 100)) {
-			return rej('invalid limit range');
-		}
-	} else {
-		limit = 10;
-	}
+	const [limit, limitErr] = it(params.limit).expect.number().range(1, 100).default(10).qed();
+	if (limitErr) return rej('invalid limit param');
 
 	// Get 'cursor' parameter
-	const cursor = params.cursor || null;
+	const [cursor, cursorErr] = it(params.cursor).expect.id().default(null).qed();
+	if (cursorErr) return rej('invalid cursor param');
 
 	// Lookup user
 	const user = await User.findOne({
-		_id: new mongo.ObjectID(userId)
+		_id: userId
 	}, {
 		fields: {
 			_id: true
@@ -61,7 +52,7 @@ module.exports = (params, me) =>
 	const query = {
 		follower_id: user._id,
 		deleted_at: { $exists: false }
-	};
+	} as any;
 
 	// ログインしていてかつ iknow フラグがあるとき
 	if (me && iknow) {
@@ -76,7 +67,7 @@ module.exports = (params, me) =>
 	// カーソルが指定されている場合
 	if (cursor) {
 		query._id = {
-			$lt: new mongo.ObjectID(cursor)
+			$lt: cursor
 		};
 	}
 
diff --git a/src/api/endpoints/users/posts.js b/src/api/endpoints/users/posts.ts
similarity index 52%
rename from src/api/endpoints/users/posts.js
rename to src/api/endpoints/users/posts.ts
index d358c4b4dd..526ed1ee1b 100644
--- a/src/api/endpoints/users/posts.js
+++ b/src/api/endpoints/users/posts.ts
@@ -3,7 +3,7 @@
 /**
  * Module dependencies
  */
-import * as mongo from 'mongodb';
+import it from '../../it';
 import Post from '../../models/post';
 import User from '../../models/user';
 import serialize from '../../serializers/post';
@@ -19,56 +19,44 @@ module.exports = (params, me) =>
 	new Promise(async (res, rej) =>
 {
 	// Get 'user_id' parameter
-	let userId = params.user_id;
-	if (userId === undefined || userId === null || userId === '') {
-		userId = null;
-	}
+	const [userId, userIdErr] = it(params.user_id, 'id');
+	if (userIdErr) return rej('invalid user_id param');
 
 	// Get 'username' parameter
-	let username = params.username;
-	if (username === undefined || username === null || username === '') {
-		username = null;
-	}
+	const [username, usernameErr] = it(params.username, 'string');
+	if (usernameErr) return rej('invalid username param');
 
 	if (userId === null && username === null) {
 		return rej('user_id or username is required');
 	}
 
-	// Get 'with_replies' parameter
-	let withReplies = params.with_replies;
-	if (withReplies == null) {
-		withReplies = true;
-	}
+	// Get 'include_replies' parameter
+	const [includeReplies, includeRepliesErr] = it(params.include_replies).expect.boolean().default(true).qed();
+	if (includeRepliesErr) return rej('invalid include_replies param');
 
 	// Get 'with_media' parameter
-	let withMedia = params.with_media;
-	if (withMedia == null) {
-		withMedia = false;
-	}
+	const [withMedia, withMediaErr] = it(params.with_media).expect.boolean().default(false).qed();
+	if (withMediaErr) return rej('invalid with_media param');
 
 	// Get 'limit' parameter
-	let limit = params.limit;
-	if (limit !== undefined && limit !== null) {
-		limit = parseInt(limit, 10);
+	const [limit, limitErr] = it(params.limit).expect.number().range(1, 100).default(10).qed();
+	if (limitErr) return rej('invalid limit param');
 
-		// From 1 to 100
-		if (!(1 <= limit && limit <= 100)) {
-			return rej('invalid limit range');
-		}
-	} else {
-		limit = 10;
-	}
+	// Get 'since_id' parameter
+	const [sinceId, sinceIdErr] = it(params.since_id).expect.id().qed();
+	if (sinceIdErr) return rej('invalid since_id param');
 
-	const since = params.since_id || null;
-	const max = params.max_id || null;
+	// Get 'max_id' parameter
+	const [maxId, maxIdErr] = it(params.max_id).expect.id().qed();
+	if (maxIdErr) return rej('invalid max_id param');
 
 	// Check if both of since_id and max_id is specified
-	if (since !== null && max !== null) {
+	if (sinceId !== null && maxId !== null) {
 		return rej('cannot set since_id and max_id');
 	}
 
 	const q = userId != null
-		? { _id: new mongo.ObjectID(userId) }
+		? { _id: userId }
 		: { username_lower: username.toLowerCase() } ;
 
 	// Lookup user
@@ -88,19 +76,19 @@ module.exports = (params, me) =>
 	};
 	const query = {
 		user_id: user._id
-	};
-	if (since !== null) {
+	} as any;
+	if (sinceId) {
 		sort._id = 1;
 		query._id = {
-			$gt: new mongo.ObjectID(since)
+			$gt: sinceId
 		};
-	} else if (max !== null) {
+	} else if (maxId) {
 		query._id = {
-			$lt: new mongo.ObjectID(max)
+			$lt: maxId
 		};
 	}
 
-	if (!withReplies) {
+	if (!includeReplies) {
 		query.reply_to_id = null;
 	}
 
diff --git a/src/api/endpoints/users/recommendation.js b/src/api/endpoints/users/recommendation.ts
similarity index 68%
rename from src/api/endpoints/users/recommendation.js
rename to src/api/endpoints/users/recommendation.ts
index 0045683a5a..c37ae4c978 100644
--- a/src/api/endpoints/users/recommendation.js
+++ b/src/api/endpoints/users/recommendation.ts
@@ -3,6 +3,7 @@
 /**
  * Module dependencies
  */
+import it from '../../it';
 import User from '../../models/user';
 import serialize from '../../serializers/user';
 import getFriends from '../../common/get-friends';
@@ -18,25 +19,12 @@ module.exports = (params, me) =>
 	new Promise(async (res, rej) =>
 {
 	// Get 'limit' parameter
-	let limit = params.limit;
-	if (limit !== undefined && limit !== null) {
-		limit = parseInt(limit, 10);
-
-		// From 1 to 100
-		if (!(1 <= limit && limit <= 100)) {
-			return rej('invalid limit range');
-		}
-	} else {
-		limit = 10;
-	}
+	const [limit, limitErr] = it(params.limit).expect.number().range(1, 100).default(10).qed();
+	if (limitErr) return rej('invalid limit param');
 
 	// Get 'offset' parameter
-	let offset = params.offset;
-	if (offset !== undefined && offset !== null) {
-		offset = parseInt(offset, 10);
-	} else {
-		offset = 0;
-	}
+	const [offset, offsetErr] = it(params.offset).expect.number().min(0).default(0).qed();
+	if (offsetErr) return rej('invalid offset param');
 
 	// ID list of the user itself and other users who the user follows
 	const followingIds = await getFriends(me._id);
diff --git a/src/api/endpoints/users/search.js b/src/api/endpoints/users/search.ts
similarity index 79%
rename from src/api/endpoints/users/search.js
rename to src/api/endpoints/users/search.ts
index b1f4537328..3fb08b0a35 100644
--- a/src/api/endpoints/users/search.js
+++ b/src/api/endpoints/users/search.ts
@@ -4,6 +4,7 @@
  * Module dependencies
  */
 import * as mongo from 'mongodb';
+import it from '../../it';
 import User from '../../models/user';
 import serialize from '../../serializers/user';
 import config from '../../../conf';
@@ -20,31 +21,16 @@ module.exports = (params, me) =>
 	new Promise(async (res, rej) =>
 {
 	// Get 'query' parameter
-	let query = params.query;
-	if (query === undefined || query === null || query.trim() === '') {
-		return rej('query is required');
-	}
+	const [query, queryError] = it(params.query).expect.string().required().trim().validate(x => x != '').qed();
+	if (queryError) return rej('invalid query param');
 
 	// Get 'offset' parameter
-	let offset = params.offset;
-	if (offset !== undefined && offset !== null) {
-		offset = parseInt(offset, 10);
-	} else {
-		offset = 0;
-	}
+	const [offset, offsetErr] = it(params.offset).expect.number().min(0).default(0).qed();
+	if (offsetErr) return rej('invalid offset param');
 
 	// Get 'max' parameter
-	let max = params.max;
-	if (max !== undefined && max !== null) {
-		max = parseInt(max, 10);
-
-		// From 1 to 30
-		if (!(1 <= max && max <= 30)) {
-			return rej('invalid max range');
-		}
-	} else {
-		max = 10;
-	}
+	const [max, maxErr] = it(params.max).expect.number().range(1, 30).default(10).qed();
+	if (maxErr) return rej('invalid max param');
 
 	// If Elasticsearch is available, search by it
 	// If not, search by MongoDB
diff --git a/src/api/endpoints/users/search_by_username.js b/src/api/endpoints/users/search_by_username.ts
similarity index 50%
rename from src/api/endpoints/users/search_by_username.js
rename to src/api/endpoints/users/search_by_username.ts
index 7fe6f34092..540c48e7ce 100644
--- a/src/api/endpoints/users/search_by_username.js
+++ b/src/api/endpoints/users/search_by_username.ts
@@ -3,8 +3,9 @@
 /**
  * Module dependencies
  */
-import * as mongo from 'mongodb';
+import it from '../../it';
 import User from '../../models/user';
+import { validateUsername } from '../../models/user';
 import serialize from '../../serializers/user';
 
 /**
@@ -18,37 +19,16 @@ module.exports = (params, me) =>
 	new Promise(async (res, rej) =>
 {
 	// Get 'query' parameter
-	let query = params.query;
-	if (query === undefined || query === null || query.trim() === '') {
-		return rej('query is required');
-	}
-
-	query = query.trim();
-
-	if (!/^[a-zA-Z0-9-]+$/.test(query)) {
-		return rej('invalid query');
-	}
-
-	// Get 'limit' parameter
-	let limit = params.limit;
-	if (limit !== undefined && limit !== null) {
-		limit = parseInt(limit, 10);
-
-		// From 1 to 100
-		if (!(1 <= limit && limit <= 100)) {
-			return rej('invalid limit range');
-		}
-	} else {
-		limit = 10;
-	}
+	const [query, queryError] = it(params.query).expect.string().required().trim().validate(validateUsername).qed();
+	if (queryError) return rej('invalid query param');
 
 	// Get 'offset' parameter
-	let offset = params.offset;
-	if (offset !== undefined && offset !== null) {
-		offset = parseInt(offset, 10);
-	} else {
-		offset = 0;
-	}
+	const [offset, offsetErr] = it(params.offset).expect.number().min(0).default(0).qed();
+	if (offsetErr) return rej('invalid offset param');
+
+	// Get 'limit' parameter
+	const [limit, limitErr] = it(params.limit).expect.number().range(1, 100).default(10).qed();
+	if (limitErr) return rej('invalid limit param');
 
 	const users = await User
 		.find({
diff --git a/src/api/endpoints/users/show.js b/src/api/endpoints/users/show.ts
similarity index 64%
rename from src/api/endpoints/users/show.js
rename to src/api/endpoints/users/show.ts
index 0eaba221cc..cae4ac0b7f 100644
--- a/src/api/endpoints/users/show.js
+++ b/src/api/endpoints/users/show.ts
@@ -3,7 +3,7 @@
 /**
  * Module dependencies
  */
-import * as mongo from 'mongodb';
+import it from '../../it';
 import User from '../../models/user';
 import serialize from '../../serializers/user';
 
@@ -18,28 +18,19 @@ module.exports = (params, me) =>
 	new Promise(async (res, rej) =>
 {
 	// Get 'user_id' parameter
-	let userId = params.user_id;
-	if (userId === undefined || userId === null || userId === '') {
-		userId = null;
-	}
+	const [userId, userIdErr] = it(params.user_id, 'id');
+	if (userIdErr) return rej('invalid user_id param');
 
 	// Get 'username' parameter
-	let username = params.username;
-	if (username === undefined || username === null || username === '') {
-		username = null;
-	}
+	const [username, usernameErr] = it(params.username, 'string');
+	if (usernameErr) return rej('invalid username param');
 
 	if (userId === null && username === null) {
 		return rej('user_id or username is required');
 	}
 
-	// Validate id
-	if (userId && !mongo.ObjectID.isValid(userId)) {
-		return rej('incorrect user_id');
-	}
-
 	const q = userId != null
-		? { _id: new mongo.ObjectID(userId) }
+		? { _id: userId }
 		: { username_lower: username.toLowerCase() } ;
 
 	// Lookup user