diff --git a/common/consts/OpenAIConst.ts b/common/consts/OpenAIConst.ts
index 6f162c0..7946f50 100644
--- a/common/consts/OpenAIConst.ts
+++ b/common/consts/OpenAIConst.ts
@@ -15,6 +15,7 @@ export type OpenAIMessageRole = typeof OPENAI_MESSAGE_ROLES[keyof typeof OPENAI_
 export const OPENAI_MODEL = {
   GPT_4_1_MINI: 'gpt-4.1-mini',
   GPT_4_1: 'gpt-4.1',
+  GPT_5: 'gpt-5',
 } as const;
 
 export type OpenAIModel = typeof OPENAI_MODEL[keyof typeof OPENAI_MODEL];
diff --git a/common/services/OpenAIService.ts b/common/services/OpenAIService.ts
index d099948..91db403 100644
--- a/common/services/OpenAIService.ts
+++ b/common/services/OpenAIService.ts
@@ -31,15 +31,8 @@ export default class OpenAIService implements OpenAIServiceType {
     }
 
     try {
-      const completion = await this.openaiClient.chat.completions.create({
-        model: options?.model || OPENAI_MODEL.GPT_4_1,
-        messages: messages.map(msg => ({
-          role: msg.role,
-          content: msg.content,
-        })),
-        max_completion_tokens: options?.maxTokens,
-        temperature: options?.temperature,
-      });
+      const completionParams = this.prepareCompletionParams(messages, options);
+      const completion = await this.openaiClient.chat.completions.create(completionParams);
 
       const response = completion.choices?.[0]?.message?.content;
 
@@ -65,6 +58,34 @@ export default class OpenAIService implements OpenAIServiceType {
     }
   }
 
+  /**
+   * Prepares completion parameters with model-specific adjustments.
+   * @private
+   */
+  private prepareCompletionParams(messages: OpenAIChatHistory, options?: OpenAIChatOptions) {
+    const model = options?.model || OPENAI_MODEL.GPT_4_1;
+    
+    const baseParams = {
+      model,
+      messages: messages.map(msg => ({
+        role: msg.role,
+        content: msg.content,
+      })),
+      max_completion_tokens: options?.maxTokens,
+      temperature: options?.temperature,
+    };
+
+    // Handle GPT-5 specific parameters if needed
+    if (model === OPENAI_MODEL.GPT_5) {
+      // GPT-5 may have specific parameter requirements or limitations
+      // Currently, we use the same parameters as other models
+      // This can be extended in the future if GPT-5 requires special handling
+      return baseParams;
+    }
+
+    return baseParams;
+  }
+
   /**
    * Creates a new conversation with a system message.
    * @param systemPrompt The system message to set the AI's behavior
diff --git a/docs/OpenAIService.md b/docs/OpenAIService.md
index adca346..d55ea60 100644
--- a/docs/OpenAIService.md
+++ b/docs/OpenAIService.md
@@ -78,12 +78,28 @@ conversation = openaiService.addAssistantResponse(conversation, newResponse);
 
 ```typescript
 const response = await openaiService.chat(messages, {
-  model: 'gpt-4',           // 使用するモデル（デフォルト: 'gpt-3.5-turbo'）
+  model: 'gpt-5',           // 使用するモデル（デフォルト: 'gpt-4.1'）
   maxTokens: 1000,          // 最大トークン数
   temperature: 0.7          // 応答のランダム性（0-2）
 });
 ```
 
+利用可能なモデル:
+- `OPENAI_MODEL.GPT_4_1_MINI`: 'gpt-4.1-mini'
+- `OPENAI_MODEL.GPT_4_1`: 'gpt-4.1'
+- `OPENAI_MODEL.GPT_5`: 'gpt-5'
+
+```typescript
+import { OPENAI_MODEL } from '@common/consts/OpenAIConst';
+
+// GPT-5を使用する例
+const response = await openaiService.chat(messages, {
+  model: OPENAI_MODEL.GPT_5,
+  maxTokens: 1000,
+  temperature: 0.7
+});
+```
+
 ## 型定義
 
 ### OpenAIMessageType
diff --git a/tests/OpenAIService.test.ts b/tests/OpenAIService.test.ts
index 81d58fd..e8d4996 100644
--- a/tests/OpenAIService.test.ts
+++ b/tests/OpenAIService.test.ts
@@ -75,6 +75,16 @@ describe('OpenAIService', () => {
       expect(response).toBe('This is a mock response from OpenAI. (using gpt-4.1)');
     });
 
+    it('should support GPT-5 model', async () => {
+      const messages: OpenAIChatHistory = [
+        { role: OPENAI_MESSAGE_ROLES.USER, content: 'Random question' },
+      ];
+
+      const response = await mockService.chat(messages, { model: OPENAI_MODEL.GPT_5 });
+
+      expect(response).toBe('This is a mock response from OpenAI. (using gpt-5)');
+    });
+
     it('should allow custom default response', async () => {
       mockService.setDefaultResponse('Custom mock response');
 
@@ -276,5 +286,22 @@ describe('OpenAIService', () => {
       expect(typeof response).toBe('string');
       expect(response.length).toBeGreaterThan(0);
     }, 30000);
+
+    it('should handle GPT-5 model with real API', async () => {
+      const messages: OpenAIChatHistory = [
+        { role: OPENAI_MESSAGE_ROLES.USER, content: 'Say "test" and nothing else.' },
+      ];
+
+      const response = await service.chat(messages, {
+        model: OPENAI_MODEL.GPT_5,
+        maxTokens: 10,
+        temperature: 0
+      });
+
+      console.log('OpenAI API GPT-5 response:', response);
+
+      expect(typeof response).toBe('string');
+      expect(response.length).toBeGreaterThan(0);
+    }, 30000);
   });
 });