diff --git a/packages/backend/src/services/drive/add-file.ts b/packages/backend/src/services/drive/add-file.ts
index ee49f6ae5..20ba24609 100644
--- a/packages/backend/src/services/drive/add-file.ts
+++ b/packages/backend/src/services/drive/add-file.ts
@@ -16,6 +16,7 @@ import { IRemoteUser, User } from '@/models/entities/user.js';
 import { driveChart, perUserDriveChart, instanceChart } from '@/services/chart/index.js';
 import { genId } from '@/misc/gen-id.js';
 import { isDuplicateKeyValueError } from '@/misc/is-duplicate-key-value-error.js';
+import { DriveFolder } from '@/models/entities/drive-folder.js';
 import { deleteFile } from './delete-file.js';
 import { GenerateVideoThumbnail } from './generate-video-thumbnail.js';
 import { driveLogger } from './logger.js';
@@ -153,7 +154,10 @@ async function save(file: DriveFile, path: string, name: string, type: string, h
  * @param type Content-Type for original
  * @param generateWeb Generate webpublic or not
  */
-export async function generateAlts(path: string, type: string, generateWeb: boolean) {
+export async function generateAlts(path: string, type: string, generateWeb: boolean): Promise<{
+	webpublic: IImage | null;
+	thumbnail: IImage | null;
+}> {
 	if (type.startsWith('video/')) {
 		try {
 			const thumbnail = await GenerateVideoThumbnail(path);
@@ -256,7 +260,7 @@ export async function generateAlts(path: string, type: string, generateWeb: bool
 /**
  * Upload to ObjectStorage
  */
-async function upload(key: string, stream: fs.ReadStream | Buffer, _type: string, filename?: string) {
+async function upload(key: string, stream: fs.ReadStream | Buffer, _type: string, filename?: string): Promise<void> {
 	const type = (_type === 'image/apng')
 		? 'image/png'
 		: (FILE_TYPE_BROWSERSAFE.includes(_type))
@@ -286,7 +290,7 @@ async function upload(key: string, stream: fs.ReadStream | Buffer, _type: string
 	if (result) logger.debug(`Uploaded: ${result.Bucket}/${result.Key} => ${result.Location}`);
 }
 
-async function deleteOldFile(user: IRemoteUser) {
+async function deleteOldFile(user: IRemoteUser): Promise<void> {
 	const q = DriveFiles.createQueryBuilder('file')
 		.where('file.userId = :userId', { userId: user.id })
 		.andWhere('file.isLink = FALSE');
@@ -387,7 +391,7 @@ export async function addFile({
 	}
 	//#endregion
 
-	const fetchFolder = async () => {
+	const fetchFolder = async (): Promise<DriveFolder | null> => {
 		if (!folderId) {
 			return null;
 		}
@@ -425,7 +429,7 @@ export async function addFile({
 	file.createdAt = new Date();
 	file.userId = user ? user.id : null;
 	file.userHost = user ? user.host : null;
-	file.folderId = folder?.id;
+	file.folderId = folder?.id ?? null;
 	file.comment = comment;
 	file.properties = properties;
 	file.blurhash = info.blurhash || null;
diff --git a/packages/backend/src/services/drive/delete-file.ts b/packages/backend/src/services/drive/delete-file.ts
index c4f7dba1c..624fada32 100644
--- a/packages/backend/src/services/drive/delete-file.ts
+++ b/packages/backend/src/services/drive/delete-file.ts
@@ -7,7 +7,7 @@ import { fetchMeta } from '@/misc/fetch-meta.js';
 import { InternalStorage } from './internal-storage.js';
 import { getS3 } from './s3.js';
 
-export async function deleteFile(file: DriveFile, isExpired = false) {
+export async function deleteFile(file: DriveFile, isExpired = false): Promise<void> {
 	if (file.storedInternal) {
 		InternalStorage.del(file.accessKey!);
 
@@ -33,7 +33,7 @@ export async function deleteFile(file: DriveFile, isExpired = false) {
 	postProcess(file, isExpired);
 }
 
-export async function deleteFileSync(file: DriveFile, isExpired = false) {
+export async function deleteFileSync(file: DriveFile, isExpired = false): Promise<void> {
 	if (file.storedInternal) {
 		InternalStorage.del(file.accessKey!);
 
@@ -63,7 +63,7 @@ export async function deleteFileSync(file: DriveFile, isExpired = false) {
 	postProcess(file, isExpired);
 }
 
-async function postProcess(file: DriveFile, isExpired = false) {
+async function postProcess(file: DriveFile, isExpired = false): Promise<void> {
 	// リモートファイル期限切れ削除後は直リンクにする
 	if (isExpired && file.userHost !== null && file.uri != null) {
 		DriveFiles.update(file.id, {
@@ -89,7 +89,7 @@ async function postProcess(file: DriveFile, isExpired = false) {
 	}
 }
 
-export async function deleteObjectStorageFile(key: string) {
+export async function deleteObjectStorageFile(key: string): Promise<void> {
 	const meta = await fetchMeta();
 
 	const s3 = getS3(meta);
diff --git a/packages/backend/src/services/drive/image-processor.ts b/packages/backend/src/services/drive/image-processor.ts
index 9b3c95af9..a85e0fbde 100644
--- a/packages/backend/src/services/drive/image-processor.ts
+++ b/packages/backend/src/services/drive/image-processor.ts
@@ -11,7 +11,7 @@ export type IImage = {
  *   with resize, remove metadata, resolve orientation, stop animation
  */
 export async function convertToJpeg(path: string, width: number, height: number): Promise<IImage> {
-	return convertSharpToJpeg(await sharp(path), width, height);
+	return convertSharpToJpeg(sharp(path), width, height);
 }
 
 export async function convertSharpToJpeg(sharp: sharp.Sharp, width: number, height: number): Promise<IImage> {
@@ -39,7 +39,7 @@ export async function convertSharpToJpeg(sharp: sharp.Sharp, width: number, heig
  *   with resize, remove metadata, resolve orientation, stop animation
  */
 export async function convertToWebp(path: string, width: number, height: number, quality = 85): Promise<IImage> {
-	return convertSharpToWebp(await sharp(path), width, height, quality);
+	return convertSharpToWebp(sharp(path), width, height, quality);
 }
 
 export async function convertSharpToWebp(sharp: sharp.Sharp, width: number, height: number, quality = 85): Promise<IImage> {
@@ -66,7 +66,7 @@ export async function convertSharpToWebp(sharp: sharp.Sharp, width: number, heig
  *   with resize, remove metadata, resolve orientation, stop animation
  */
 export async function convertToPng(path: string, width: number, height: number): Promise<IImage> {
-	return convertSharpToPng(await sharp(path), width, height);
+	return convertSharpToPng(sharp(path), width, height);
 }
 
 export async function convertSharpToPng(sharp: sharp.Sharp, width: number, height: number): Promise<IImage> {
diff --git a/packages/backend/src/services/drive/internal-storage.ts b/packages/backend/src/services/drive/internal-storage.ts
index fa8f0ef83..e76749d6c 100644
--- a/packages/backend/src/services/drive/internal-storage.ts
+++ b/packages/backend/src/services/drive/internal-storage.ts
@@ -10,25 +10,25 @@ const _dirname = dirname(_filename);
 export class InternalStorage {
 	private static readonly path = config.internalStoragePath || Path.resolve(_dirname, '../../../../../files');
 
-	public static resolvePath = (key: string) => Path.resolve(InternalStorage.path, key);
+	public static resolvePath = (key: string): string => Path.resolve(InternalStorage.path, key);
 
-	public static read(key: string) {
+	public static read(key: string): fs.ReadStream {
 		return fs.createReadStream(InternalStorage.resolvePath(key));
 	}
 
-	public static saveFromPath(key: string, srcPath: string) {
+	public static saveFromPath(key: string, srcPath: string): string {
 		fs.mkdirSync(InternalStorage.path, { recursive: true });
 		fs.copyFileSync(srcPath, InternalStorage.resolvePath(key));
 		return `${config.url}/files/${key}`;
 	}
 
-	public static saveFromBuffer(key: string, data: Buffer) {
+	public static saveFromBuffer(key: string, data: Buffer): string {
 		fs.mkdirSync(InternalStorage.path, { recursive: true });
 		fs.writeFileSync(InternalStorage.resolvePath(key), data);
 		return `${config.url}/files/${key}`;
 	}
 
-	public static del(key: string) {
+	public static del(key: string): void {
 		fs.unlink(InternalStorage.resolvePath(key), () => {});
 	}
 }
diff --git a/packages/backend/src/services/drive/s3.ts b/packages/backend/src/services/drive/s3.ts
index 80e34be95..f698d9869 100644
--- a/packages/backend/src/services/drive/s3.ts
+++ b/packages/backend/src/services/drive/s3.ts
@@ -3,7 +3,7 @@ import S3 from 'aws-sdk/clients/s3.js';
 import { Meta } from '@/models/entities/meta.js';
 import { getAgentByUrl } from '@/misc/fetch.js';
 
-export function getS3(meta: Meta) {
+export function getS3(meta: Meta): S3 {
 	const u = meta.objectStorageEndpoint != null
 		? `${meta.objectStorageUseSSL ? 'https://' : 'http://'}${meta.objectStorageEndpoint}`
 		: `${meta.objectStorageUseSSL ? 'https://' : 'http://'}example.net`;
diff --git a/packages/backend/src/services/drive/upload-from-url.ts b/packages/backend/src/services/drive/upload-from-url.ts
index 65a062faf..a815d8374 100644
--- a/packages/backend/src/services/drive/upload-from-url.ts
+++ b/packages/backend/src/services/drive/upload-from-url.ts
@@ -58,7 +58,7 @@ export async function uploadFromUrl({
 			sensitive,
 		});
 		logger.succ(`Got: ${driveFile.id}`);
-		return driveFile!;
+		return driveFile;
 	} catch (e) {
 		logger.error(`Failed to create drive file: ${e}`, {
 			url,