Sept. 14 (Bloomberg) -- Shares of Mixi Inc., Japan's largest online community, doubled on the first day of trading, making 30- year-old founder Kenji Kasahara an instant billionaire.

Investors bid 3.15 million yen ($26,800) for each share, compared with the initial offering price of 1.55 million yen. The stock didn't trade because there were offers to buy more shares than the Tokyo-based company sold.

Kasahara, whose 65 percent stake soared in value to $1.2 billion, is riding a global wave of customers for socializing Web sites such as News Corp.'s MySpace.com. Mixi's members can swap gossip and cooking tips or use key-word searches to find long- lost friends or people with shared interests.

``They're adding more than 10,000 people a day,'' said Oliver Cox, an Internet analyst at Macquarie Securities Ltd. in Tokyo. ``There's some question how long this can be sustained, but everything looks good at this point.''

Five million people were using Mixi as of July, up from 3.4 million in March, Kasahara said in an interview earlier this month. MySpace.com has more than 100 million member profiles.

Kasahara predicted that sales at his site will more than double this fiscal year to 4.8 billion yen, with pretax profit from operations rising 89 percent to 1.7 billion yen. He owns 45,700 Mixi shares.

``I'm very grateful,'' Kasahara said at a press briefing held at the Tokyo Stock Exchange after the close of trading. ``I feel a heavy sense of responsibility to the shareholders and will do my best to fulfill their expectations.''

Tokyo University

A business graduate of Tokyo University, Kasahara started Mixi, originally called E-Mercury, in 1999. The company won Yahoo Japan Corp.'s Web Site of the Year award in 2005.

Mixi makes 80 percent of its profit from advertising and the rest from service fees. Its subscribers accounted for 47 percent of Japanese who have joined networking sites, according to a communications ministry study.

``I'm expecting the stock price to keep on rising,'' said Minoru Takada, who helps oversee $61 billion at Mitsubishi UFJ Asset Management Co. in Tokyo. ``Mixi is a new company with expanding earnings and high recognition.'' He declined to say whether he bought Mixi shares.

Mixi shares trade on the Tokyo Stock Exchange's Mothers market, which focuses on start-up companies. Investors offered to buy 8,616 shares as of the close of trading in Tokyo, compared with sell orders for 3,505 shares. Buy and sell orders have to match before trades are registered

In its IPO, Mixi offered 7,100 shares, including an extra 500 shares to meet demand.

Shares of Gomez Consulting Co., a Tokyo-based Web site designer, surged 161 percent since they started trading on Aug. 16, the biggest return for a Japanese IPO this year, according to data compiled by Bloomberg.

To contact the reporter for this story: Makiko Suzuki in Tokyo at msuzuki13@bloomberg.net; John Brinsley in Tokyo at jbrinsley@bloomberg.net

Last Updated: September 14, 2006 05:25 EDT