Mercari Inc. priced its initial public offering at the top of the range to sell ¥130.5 billion ($1.2 billion) worth of shares, a valuation that's set to make its 40-year-old founder a billionaire.

The online marketplace plans to sell 43.5 million shares at ¥3,000 apiece, including the sale of additional shares via a greenshoe allotment, the company said Monday. That puts the fortune of founder Shintaro Yamada, who owns almost a third of the company, at ¥115 billion. The indicated IPO price gives Mercari a market value of ¥406 billion, and the stock will begin trading in Tokyo on June 19.

Yamada, whose previous company was acquired by Zynga Inc., built the flea market by focusing on smartphone users, while Yahoo Japan Corp.'s dominant auctions website is mainly a desktop-based marketplace. The app, which debuted in 2013, now has more than 10 million monthly active customers. Three years after its release, Mercari became Japan's first startup to reach the so-called unicorn valuation of $1 billion or more.