How can i purchase an app of japan store without change my itunes store? i have balance in my US ac so i cant change my stores, but the app i want to purchase in jp store is a free app.... i know apple dun allow balance spend in other countriesbutitisfree

You cannot use the Japanese iTunes Store unless are physically present there and have a Japanese payment method. This applies even for free apps; you cannot get them from the Japanese iTunes Store if you are in the US.

If you are in Japan, you can set up a new iTunes Store account just to get free apps. See:

http://support.apple.com/kb/HT2534

Read the steps carefully as the order in which you follow them is critical. Note that you can do this only when creating a new Apple ID. You cannot use an existing ID.

If you have moved to Japan and wish to change the country of your account permanently, you can contact the iTunes Store and ask them to clear your remaining balance. You will need to be able to provide a Japanese payment method, though.

Regards.