According to the MCF, Japan’s market for mobile gaming reached roughly US$5.1 billion in 2012.
I translated the key information of the report into English.
According to the MCF, the market for mobile commerce and content combined in Japan was sized at 2.3507 trillion yen in 2012 (US$24.4 billion), up a very handsome 23% from 2011.
Within that US$24.4 billion market, mobile content accounted for US$8.8 billion:
The market just for mobile content in Japan in 2012 was still bigger on feature phones (US$5 billion), but it contracted by 27% when compared to a year before.
That difference was overcompensated by the respective market on smartphones, which grew 4.6-fold year-on-year to reach US$3.8 billion:
What about games inside this mobile content market?
The size of the market for gaming on feature phones in Japan in 2012 reached around US$2.4 billion: roughly US$2 billion for social games, and another US$410 million for mobile games (titles that are not distributed through social gaming networks like GREE or Mobage).
Observe how both sub-segments shrank year-on-year on feature phones:
On the smartphone side, the gaming segment reached US$2.7 billion for all game-related content (here, the MCF doesn’t make a special distinction).
This segment on smartphones grew 5.4-fold year-on-year:
As a summary, the Japanese market for mobile gaming in 2012 reached US$5.1 billion overall, according to the MCF: US$2.4 billion on feature phones, and US$2.7 billion on smartphones.