Questions have been raised over the legality of a portion of a popular online game in which players dice for virtual currency, since winnings can be exchanged for real money elsewhere on the Internet.

"Dragon Quest X" is enormously popular, selling 500,000 copies in the month since its release. In the game, players are able to invite other players to dice games using virtual currency that is supposed to be used only inside the game.

In reality, the virtual cash can be openly exchanged for actual currency at real money trade (RMT) websites. This turns the play gambling during the game into unauthorized betting, which is prohibited by the Penal Code, according to legal experts.

RMT sites allow gamers to buy and sell virtual currency or items used in video games for real money. The service is available at sites operated by Internet auction companies and firms that specialize in the practice.

The service has been widely available for about 10 years, and is used by a large number of online gamers to increase the levels they can reach in games or to obtain rare items. The RMT market is estimated to be worth at least 10 billion yen, but is not regulated by law.