I'm using US Apple ID and got a gift card from Japan. I can't redeem it. How can I redeem the gift card?
You can switch the country of your iTunes account, but only if you have a valid Japanese credit card with a billing address in Japan.
Note that by doing this, you will no longer be able to update apps purchased in the US store, unless they are available in the JP store and you download them again from that store. (It should be free to download them if available, but they may not be available in the JP store store.)
Unless you are going to be living in Japan, it's probably better to try and sell the JP gift card and keep your account country as US.