Hi Suguru Iwata,
What do you mean by Database is not working?
I am giving a suggestion based on my assumption !!
Is the Oracle database you are connecting a database created by synchronizing with your project's Domain model or is it an existing database created manually?
The schema will be different for your Mendix project and a database created manually.
If you want data from an existing Database in your Mendix Project, I suggest you, let Mendix create your database and then use the database Reflection module in your project to fetch data from your other database into your Mendix DB for the first time and use Database connector module to fetch data whenever needed.