Japan passed the U.S. this year to become the top-grossing market for combined Google Play and Apple App Store mobile revenue. Revenue from mobile games in Japan has nearly quadrupled in the past year, according to a recent report released by the mobile market intelligence agency App Annie. The lion's share of that is spent in Google's Play Store, which in Japan is actually neck-and-neck with Apple's App Store when it comes to monthly revenue. That may be due to the fact that -- according to the App Annie report -- many Japanese wireless providers allow subscribers to charge their Google Play purchases to their phone bill, rather than having to rely on paying through a separate Google account with credit cards or cash. Google Play games also dominate because Apple's share of the Japanese smartphone market is smaller than, say, the U.S. or Europe, though that's likely to change now that Japanese telecom DoCoMo has begun offering the iPhone to its customers. It's worth noting that the top earners in the Japanese mobile market are local companies like GungHo Online, Namco Bandai and Sega. The mobile gaming scene in Japan is unique -- freemium titles like Puzzle and Dragons or collectible card games do well, and many popular games are buttressed by TV ad campaigns and other traditional marketing techniques. Given the success that foreign mobile developers like Supercell have reportedly had in selling their games in Japan after partnering with Japanese developers, cross-promotion (if not full-on localization) seems key to success in this rapidly-growing market.