Mizuho Financial Group is planning to buy out its brokerage and trust bank units, a source familiar with the matter said on Saturday, a deal that could be worth around $4 billion as the lender tries to grow beyond sluggish commercial banking business in Japan.

Japan's second-largest bank by assets is planning to buy out minority shareholders in Mizuho Securities <8606.T>, Mizuho Trust & Banking <8404.T>, Mizuho Investors Securities <8607.T>, said the source, who was not authorized to talk about the matter publicly.

Mizuho is expected to carry out the buyout around October after getting approval at an annual shareholders meeting scheduled in June, the source said.

Mizuho and rival top banks Mitsubishi UFJ Financial Group <8306.T> and Sumitomo Mitsui Financial Group <8316.T> are trying to strengthen their investment banking and other operations as they face weak prospects in their core lending business in Japan.

Mizuho aims to consolidate its group management for better control and synergy among units by buying out the minority shareholders in these companies.

Mizuho has 75 percent in Mizuho Trust in terms of voting rights, 60 percent in Mizuho Securities and 67 percent in Mizuho Investors Securities. Their minority shareholdings are worth roughly 300 billion yen ($3.7 billion) in terms of voting rights. ($1=81.72 Yen)

(Reporting by Taiga Uranaka, additional reporting by Santosh Nadgir in BANGALORE; Editing by Sugita Katyal3)

Japan