Q: How to use my iphone5S in Japan

Hey new step,

Thanks for the question. It is always best to consult with your carrier when planning to travel internationally. In addition to that, the following resource may assist you:

iPhone and iPad: Using cellular networks while in another country

http://support.apple.com/kb/HT1807

Many carriers offer international voice and data plans. Always contact your current carrier for details about international-roaming countries, SIMs, rates, and policies before you travel to another country. While you may not need an international plan to use voice or data roaming on your iPhone or iPad, your carrier may charge additional fees or higher data rates if you don't have an international plan. 

Thanks,

Matt M.