Players can win a grand prize, a rare virtual item, after purchasing a certain number of required items.
The Consumer Affairs Agency has concluded that a system used by some online game operators on social networking service (SNS) websites constitutes a violation of a law that bans certain types of sales methods, according to sources close to the agency.
In such online games, known as "kompu gacha" (complete gacha), players can win a grand prize, a rare virtual item, after purchasing a certain number of required items.
The agency began investigating the game services after receiving complaints about extremely high charges imposed on players of such games on SNS websites such as Gree and Mobage.
The agency will soon release its views on the issue, the sources said.
The agency also plans to ask companies that offer such games to stop using the questionable sales method.
If companies fail to comply with the request, the agency will issue a correction order with punitive measures in line with the law against unjustifiable premiums and misleading representation.
The market for online mobile-phone games is worth about 250 billion yen (S$3.8 billion), with kompu gacha games having become the main revenue source for SNS operators of such services.
However, the kompu gacha games have often been criticized for taking advantage of users' gambling spirit.
Some parents complained their children had become addicted and were billed for exorbitant charges.
The original gacha games were inspired by automatic vending machines that sell capsules containing toys.
However, instead of buying real items, online gacha players purchase virtual items for hundreds of yen, which they then use for various purposes.
The players do not know in advance which item they will get, giving the game the feel of a lottery.
In kompu gacha, an advanced form of gacha, players can obtain rare items once they collect a certain combination of items available in gacha games.
This system was introduced in online games by SNS website operators around last year.
Popular kompu gacha games include Mobage's "The Idolm@ster Cinderella Girls," and Gree's "Tanken Dori Rando."
According to the agency and other sources, there were only five complaints or inquiries concerning the high charges for gacha games in fiscal 2010.
However, the number surged to 58 in fiscal 2011.
In one case, a middle school boy was charged more than 400,000 yen in just one month, while a primary school boy racked up 120,000 yen in charges over three days playing the games.
The law on unjustifiable premiums prohibits a sales method called "cards combination" for systems that offer prizes by lottery.
Under this method, a business sells cards whose content is unknown at the time of purchase. After the consumer collects designated cards, they can trade them in for a prize.
The agency concluded that kompu gacha games employ this business method.
The agency plans to ask Social Game Platform Renraku Kyogikai, a liaison council of six social game makers, including Gree and DeNA Co., which operates the Mobage site, to stop offering kompu gacha games.
"We'll give an appropriate response after receiving an official request," a Gree spokesperson said. A DeNA spokesperson declined to comment.
Social games are provided to members of SNS websites, on which people interact with their friends and others via the Internet.
In many of these types of games, players can choose to cooperate with or compete against each other.
Though the games are free, in principle, aside from Internet access fees, users have the option to purchase such items as virtual weapons and clothing if they want to have an advantage in the game.