A shareholder of Yamaichi Securities Co. filed a lawsuit Monday against former executives of the firm, demanding they return about 6 billion yen to the failed brokerage.The lawsuit, filed with the Tokyo District Court, is the first legal action by shareholders against Yamaichi over its window dressing.The litigation accuses former Yamaichi Chairman Tsugio Yukihira, 67, and seven other former board members of concealing off-the-book debts and causing damage to the company by illegally paying dividends without profits to justify the payments. Yukihira is currently on trial for allegedly violating the Commercial Code.Former Yamaichi President Atsuo Miki, 63, is also named in the lawsuit. He is facing another shareholder lawsuit over his involvement in the brokerage's illicit payoffs to a "sokaiya" corporate extortionist.The company's off-the-book transactions allowed Yamaichi to underreport losses of more than 156 billion yen and produce a fund surplus to pay 6 billion yen in annual dividends to shareholders, according to the lawsuit filed Monday.Yamaichi went bankrupt last November with off-the-book debts of 260 billion yen.