* DeNA being probed for anti-competitive practices
* Watchdog checking if developer treated unfairly -source
* Shares tumble 10.5 pct vs. 0.8 pct rise in Nikkei 225 (Recasts, adds details)
TOKYO, Dec 8 Japanese social networking site operator DeNA Co (2432.T) said on Wednesday it is being investigated by regulators on suspicion of unfair competitive practices, sending its shares down more than 10 percent.
DeNA operates a popular mobile-only social game site, called "Mobage-town", which has 20 million users in Japan.
A company source told Reuters that the Fair Trade Commission is looking into whether DeNA had treated a mobile game developer unfairly after the developer created a game for a rival website owned by Gree Inc (3632.T).
DeNA spokesman Tetsuhiro Kaneko confirmed there was an investigation but declined to elaborate further.
DeNA shares slid to 2,417 yen by Wednesday afternoon compared with a 0.8 percent rise in the benchmark Nikkei 225 index .N225. It was the most actively traded stock on the main board. (Reporting by Reiji Murai, James Topham and Mariko Katsumura; Editing by Michael Watson)