Some Japanese apps like Taiko no Tatsujin Plus and jubeat plus are free to download once you create a Japanese iTunes account. However there are only a few songs bundled in the free apps. You need to purchase more songs inside the app.

The problem is you do not have any credit card of which the billing address is in Japan, it is too troublesome to get Japan iTunes store gift card and you cannot use another US iTunes account to do in-app purchase in app you downloaded with the Japan iTunes account.

I just found a way to work around these. The following is all you need:

1. An email address which is not associated with any iTunes store account
2. A valid credit card.

Here are the steps:

• Open iTunes on your computer
• Go to iTunes store
• Sign out from your current iTunes account if you have signed in
• Scroll to the bottom of the page and click the flag to change the country of the iTunes store
• Find “日本” (Japan, it’s at the end of the page) and click it, it will bring you to the Japanese iTunes store
• Go to App Store, find any free app and try to download that. It will ask you to sign in or create a new account.
• Click “create new account”. Create the account as usual (email, password, security question and date of birth). Next you’ll need to enter payment information. You can select “none” because you got here by trying to download a free app. In billing address, you need to enter an address in Japan. It is easy to find one on the internet.
• After that you need to go to your mailbox to verify your email address.
• Once your email is verified, you can go back to iTunes.  Select “Store” and then “Authorize This Computer” with your newly created iTunes account. Download jubeat plus or Taiko no Tatsujin Plus and sync the App to your iOS device.
• And then change your account region by Selecting “Store” => “View My Account” and then select “Change Country or Region”. Select your current country or region. You will need to enter your payment information.
• Go to your iOS device and go to settings => store. Login with your new account.
• Go to the store of jubeat plus or Taiko no Tatsujin Plus. Try purchasing download content. It will tell you to verify your payment information. After that you will be able to do the in-app purchase.

Downside of this workaround is that you will not be able to change back the region to Japan to download any other apps only available in Japan because it will require payment information to do so.