As some of you many know, we have 3 major mobile companies in Japan.
1) Docomo
2) Softbank
3) au
According to the recent data at Mobilemarketshare, 45.5% Docomo, 29% AU and 25.5% Softbank in Dec 2013. Although Docomo is dominant in Japanese mobile market, you can find many Japanese use iPhone.

docomo-au-sb

To have iPhone in Japan, we had to shift from your current mobile network to Softbank. Softbank was the only mobile company sold iPhone. Then the situation changed a few years ago, now you can have iPhone via all 3 companies.

An article mentions about a mobile market in Japan.
Android ends the year on top but Apple scores in key markets

“In Japan, consumers’ desire for all things Apple continued into the final quarter of 2013, with iOS taking 68.7% share of smartphone sales. Apple’s deal to sell iPhones through Japan’s largest carrier, NTT DoCoMO, has proved an unarguable success with Apple’s share on the carrier reaching 58.1% in the fourth quarter compared with 91.7% on Softbank and 63.7% on AU KDDI.”

Wow, Japanese love Apple!!
If you would like to enter Japanese market, you would need to consider the market share. As usual, you must translate all your App in Japanese. If your products were only in English, you would never get many customers in Japan.

To understand Japanese mobile market share, you can find much information here.
Japanese mobile phone market share