Update (as of 13 March 2015): these instructions still work for iOS 8.2. Happy downloading!
There are tons of fantastic free apps on the Japanese App Store that don’t show up when you search from an overseas iTunes account. From navigation tools to puzzle and card games, Japanese iTunes account holders are spoilt for choice. However, there is an easy way to get around that virtual barrier — all you need is an existing iTunes account and a bit of Google-fu.
First, flick over to your iPhone or iPad’s Settings, scroll down to the iTunes and App Store tab, and tap on it. Tap on the Apple ID tab, and an Apple ID pop up will appear. Tap on the View Apple ID tab to jump to the Account screen.
On the Account screen, tap on the Country/Region tab — it should take you to the Change Country/Region screen. Tap on the Change Country/Region as well. In the next screen, tap on the Store tab and a roll tab will appear. Scroll to the very bottom, and Japan (日本) should show up. Select that option.
When Japan has been set as the Country or Region, tap on Next. It will take you to the iTunes Store TOS — tap on Agree for the pop up to appear. Tap on Agree again and you will be taken to the Billing Information page.
Tap on None for your payment settings if you only intend to download free apps, otherwise input a Credit Card number (note that the system will only accept credit cards issued in Japan). Input your name, Japanese address, and Japanese telephone number (for the latter two, put your Google-fu to good use). Hit Next and you’re done!
You can check your Account settings screen again to confirm if your country has been set to Japan (the Japanese flag should be displayed). Enjoy your new Japanese iTunes account! You are now free to download as many apps as you like that were previously only available to Japanese iTunes users :3
You can switch back to your original country’s store without having the Japanese game or app disappear from your device. You will, however, need to switch back to the Japanese store when the Japanese app requires an update.