diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7feb89b546..1113374595 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,7 @@
 ### Server
 - Fix: 時間経過により無効化されたアンテナを再有効化したとき、サーバ再起動までその状況が反映されないのを修正 #12303
 - Fix: ロールタイムラインが保存されない問題を修正
+- Fix: api.jsonの生成ロジックを改善 #12402
 
 ## 2023.11.1
 
diff --git a/packages/backend/generate_api_json.js b/packages/backend/generate_api_json.js
new file mode 100644
index 0000000000..5819c60a5f
--- /dev/null
+++ b/packages/backend/generate_api_json.js
@@ -0,0 +1,8 @@
+import { loadConfig } from './built/config.js'
+import { genOpenapiSpec } from './built/server/api/openapi/gen-spec.js'
+import { writeFileSync } from "node:fs";
+
+const config = loadConfig();
+const spec = genOpenapiSpec(config);
+
+writeFileSync('./built/api.json', JSON.stringify(spec), 'utf-8');
\ No newline at end of file
diff --git a/packages/backend/package.json b/packages/backend/package.json
index 3b029a49d2..18c7818dcc 100644
--- a/packages/backend/package.json
+++ b/packages/backend/package.json
@@ -23,7 +23,8 @@
 		"jest-and-coverage": "cross-env NODE_ENV=test node --experimental-vm-modules --experimental-import-meta-resolve node_modules/jest/bin/jest.js --coverage --forceExit",
 		"jest-clear": "cross-env NODE_ENV=test node --experimental-vm-modules --experimental-import-meta-resolve node_modules/jest/bin/jest.js --clearCache",
 		"test": "pnpm jest",
-		"test-and-coverage": "pnpm jest-and-coverage"
+		"test-and-coverage": "pnpm jest-and-coverage",
+		"generate-api-json": "node ./generate_api_json.js"
 	},
 	"optionalDependencies": {
 		"@swc/core-android-arm64": "1.3.11",
diff --git a/packages/backend/src/server/api/endpoints/federation/show-instance.ts b/packages/backend/src/server/api/endpoints/federation/show-instance.ts
index 71eec11235..781c15e742 100644
--- a/packages/backend/src/server/api/endpoints/federation/show-instance.ts
+++ b/packages/backend/src/server/api/endpoints/federation/show-instance.ts
@@ -16,12 +16,9 @@ export const meta = {
 	requireCredential: false,
 
 	res: {
-		oneOf: [{
-			type: 'object',
-			ref: 'FederationInstance',
-		}, {
-			type: 'null',
-		}],
+		type: 'object',
+		optional: false, nullable: true,
+		ref: 'FederationInstance',
 	},
 } as const;
 
diff --git a/packages/backend/src/server/api/openapi/gen-spec.ts b/packages/backend/src/server/api/openapi/gen-spec.ts
index 4f972d3f7e..30bf6b8b3e 100644
--- a/packages/backend/src/server/api/openapi/gen-spec.ts
+++ b/packages/backend/src/server/api/openapi/gen-spec.ts
@@ -4,7 +4,7 @@
  */
 
 import type { Config } from '@/config.js';
-import endpoints from '../endpoints.js';
+import endpoints, { IEndpoint } from '../endpoints.js';
 import { errors as basicErrors } from './errors.js';
 import { schemas, convertSchemaToOpenApiSchema } from './schemas.js';
 
@@ -33,16 +33,17 @@ export function genOpenapiSpec(config: Config) {
 			schemas: schemas,
 
 			securitySchemes: {
-				ApiKeyAuth: {
-					type: 'apiKey',
-					in: 'body',
-					name: 'i',
+				bearerAuth: {
+					type: 'http',
+					scheme: 'bearer',
 				},
 			},
 		},
 	};
 
-	for (const endpoint of endpoints.filter(ep => !ep.meta.secure)) {
+	// 書き換えたりするのでディープコピーしておく。そのまま編集するとメモリ上の値が汚れて次回以降の出力に影響する
+	const copiedEndpoints = JSON.parse(JSON.stringify(endpoints)) as IEndpoint[];
+	for (const endpoint of copiedEndpoints.filter(ep => !ep.meta.secure)) {
 		const errors = {} as any;
 
 		if (endpoint.meta.errors) {
@@ -79,6 +80,13 @@ export function genOpenapiSpec(config: Config) {
 			schema.required = [...schema.required ?? [], 'file'];
 		}
 
+		if (schema.required && schema.required.length <= 0) {
+			// 空配列は許可されない
+			schema.required = undefined;
+		}
+
+		const hasBody = (schema.type === 'object' && schema.properties && Object.keys(schema.properties).length >= 1);
+
 		const info = {
 			operationId: endpoint.name,
 			summary: endpoint.name,
@@ -92,17 +100,19 @@ export function genOpenapiSpec(config: Config) {
 			} : {}),
 			...(endpoint.meta.requireCredential ? {
 				security: [{
-					ApiKeyAuth: [],
+					bearerAuth: [],
 				}],
 			} : {}),
-			requestBody: {
-				required: true,
-				content: {
-					[requestType]: {
-						schema,
+			...(hasBody ? {
+				requestBody: {
+					required: true,
+					content: {
+						[requestType]: {
+							schema,
+						},
 					},
 				},
-			},
+			} : {}),
 			responses: {
 				...(endpoint.meta.res ? {
 					'200': {
@@ -118,6 +128,11 @@ export function genOpenapiSpec(config: Config) {
 						description: 'OK (without any results)',
 					},
 				}),
+				...(endpoint.meta.res?.optional === true || endpoint.meta.res?.nullable === true ? {
+					'204': {
+						description: 'OK (without any results)',
+					},
+				} : {}),
 				'400': {
 					description: 'Client error',
 					content: {
@@ -190,6 +205,7 @@ export function genOpenapiSpec(config: Config) {
 		};
 
 		spec.paths['/' + endpoint.name] = {
+			...(endpoint.meta.allowGet ? { get: info } : {}),
 			post: info,
 		};
 	}
diff --git a/packages/backend/src/server/api/openapi/schemas.ts b/packages/backend/src/server/api/openapi/schemas.ts
index 1a1d973e56..2716f5f162 100644
--- a/packages/backend/src/server/api/openapi/schemas.ts
+++ b/packages/backend/src/server/api/openapi/schemas.ts
@@ -7,10 +7,16 @@ import type { Schema } from '@/misc/json-schema.js';
 import { refs } from '@/misc/json-schema.js';
 
 export function convertSchemaToOpenApiSchema(schema: Schema) {
-	const res: any = schema;
+	// optional, refはスキーマ定義に含まれないので分離しておく
+	// eslint-disable-next-line @typescript-eslint/no-unused-vars
+	const { optional, ref, ...res }: any = schema;
 
 	if (schema.type === 'object' && schema.properties) {
-		res.required = Object.entries(schema.properties).filter(([k, v]) => !v.optional).map(([k]) => k);
+		const required = Object.entries(schema.properties).filter(([k, v]) => !v.optional).map(([k]) => k);
+		if (required.length > 0) {
+			// 空配列は許可されない
+			res.required = required;
+		}
 
 		for (const k of Object.keys(schema.properties)) {
 			res.properties[k] = convertSchemaToOpenApiSchema(schema.properties[k]);