* Nomura shares down 0.8 percent, broader market up
* Nomura staff source of leak in 2nd insider case-sources
* Regulator poised to seek fine against S'tomo Mitsui Trust-sources
TOKYO, May 28 (Reuters) - Shares of Nomura Holdings edged lower in early Monday trade after sources said Japan's largest broker was linked to a second insider trading case involving a fund management arm of Sumitomo Mitsui Trust Holdings
Nomura's stock was down 0.8 percent at 258 yen as of 0021 GMT after falling as low as 256 yen. It was underperforming the benchmark Nikkei average, which gained 0.3 percent.
The Securities and Exchange Surveillance Commission (SESC) found that a fund manager in the Sumitomo Mitsui group sold shares of Mizuho Financial Group with knowledge of the lender's stock offering in 2010 before it became public, sources told Reuters on Saturday.
The SESC believes an employee of Nomura, which was an underwriter on the 780 billion yen ($9.8 billion) Mizuho offering, provided that tip-off, according to the sources, who were not authorised to speak to media about the matter.
The case is the second to involve Nomura since the regulator launched an industry-wide probe into suspicious trading around a series of share offerings in 2009 and 2010 that have tainted the reputation of Japan's capital markets.
The regulator believes a Nomura employee was also the source of a leak of information about a separate offering by energy firm Inpex. The SESC recommended a fine against the Sumitomo Mitsui unit for that incident in March.
The latest case could increase the chance that Nomura will face sanctions from an industry-wide probe into suspicious trading around a series of public share offerings that have tainted the reputation of Japan's capital markets.
It will also add to the pressure on CEO Kenichi Watanabe, who had vowed to clean up the broker's internal controls following an insider trading scandal in 2008.
Shares of Sumitomo Mitsui Trust Holdings were flat at 206 yen.