Unable to download an iOS app from iTunes Japan when I am using my Apple account registered in the USA
You can try following this procedure (Youtube Video) to create an Apple ID in iTunes without a Credit Card, substituting US for Japan.

You need to follow these steps to create an Apple ID with a U.S. address but without requiring a credit card:


Launch iTunes and switch to the iTunes store. Make sure you are not
logged in.
Scroll to the bottom and change your country to United States.
Next go to App store and download any of the free apps.


iTunes will now prompt you to create a new Apple account and this time, you’ll definitely see the ‘none’ option on the Payments screen.

Source.