How do I use my iPhone in Japan?

Hello!I will be staying in Japan for a year and I was wondering if I can use my iPhone 4S with a prepaid card bought in japan?And how do I actually buy one?I've heard that most companies don't sell such cards without a phone.And if I am to buy a phone,can I buy an iPhone with prepaid card?I was thinking about iPhone 5S if they have good pricing offer.Because making a contract lasts 2 years from what I heard...

You can get a monthly plan at a virtual carrier like b-mobile, OCN or so-net, but be warned that the data will be very slow on most of them. The best virtual carrier seems to be UQ (a subsidiary of KDDI, which is also the parent company of Softbank), but it doesn't play well with iPhones. http://www.uqmobile.jp/en/

Sorry, KDDI is the parent company of au, not Softbank.

Yes, you should be able to use your iPhone in Japan provided that the phone is unlocked and you are able to obtain a Japanese SIM card, however please be aware that you may experience compatibility problems like I do if your phone is not designed for the cellphone network in Japan. Currently I am using an iPhone 5S that I bought from New Zealand with a Japanese b-mobile SIM card, but my phone can only connect to the 3G network due to a compatibility issue, so I can not receive internet data at full speed.

Virtual carriers like b-mobile sell sim cards on their Japanese website. You can also order them online from Amazon Japan or buy them in store at Japanese electronic stores such as Bic Camera and Yadobashi Camera.