* Mothers stocks stuck 90 pct below IPO price face delisting
* New delisting rules become effective from November
* Companies already listed on Mothers exempt from new rules (Adds details, background, byline)
By Junko Fujita
TOKYO, Aug 25 (Reuters) - The Tokyo Stock Exchange said on Tuesday it plans to tighten its rules for listings on the bourse's Mothers market for start-ups from November.
The TSE, the world's second-largest exchange, will delist a Mothers stock that stays more than 90 percent below its initial public offering price for a nine-month period within three years of its market debut.
The new rules will apply to companies that debut on the Mothers market after the rules are enacted in November, Tokyo Stock Exchange Chief Executive Atsushi Saito told reporters.
Companies already listed on the market will be exempt.
The TSE set up the Mothers market in 1999 to encourage smaller firms to go public, but investors have been concerned it has sometimes helped companies not ready for public trade to list too easily.
The latest regulation tightening comes as the TSE struggles to improve its competitiveness and attract foreign investors and companies to return to the Japanese market.
The TSE has already tightened rules on third-party allotments, requiring a limit on the number of shares issued so as to avoid extreme dilution. Those rules came into effect on Monday.
The bourse has also set up a new professionals-only stock market, Tokyo AIM, under a joint venture that is 51 percent owned by the TSE and the rest by the London Stock Exchange.
Tokyo AIM allows issuers to report under U.S. GAPP, IFRS or Japan GAPP accounting standards. Companies can report either in English or in Japanese, and there is no obligation to report quarterly.
Foreign investors sold a net 3.7 trillion yen ($39.36 billion) worth of Japanese stocks last year, becoming net sellers for the first time since 2003 and helping drive the benchmark Nikkei average down more than 40 percent.
At its peak there were 120 overseas companies listed on the TSE, but that has dwindled to just 15. ($1=94.00 Yen) (Editing by Michael Watson)