Mobile OS browsing share in Japan is currently dominated by Android devices according to the study conducted by Samurai Factory last September, 2012. The Android Operating System has achieved a 62.48% of the total, compared to the 37.52% obtained by iOS.
The smartphone browser breakdown is not that clear though, because many Android browsers identify themselves as iPhone clients when browsing the internet (to benefit from the mobile version of the websites).
It is also interesting to note the percentage earned by mobile platforms (30%) compared to desktop browsers (70%). Considering that the "desktop" group also includes the tablets, we have a scenario where mobility is gaining ground rapidly.
Source: Samurai Factory