Mizuho Financial Group Inc.'s trust banking arm has its sights set on exactly where the wealth is in Japan: the $11 trillion held by the nation's elderly.

The company has begun selling a type of trust that gives rich retirees more options for managing their money, and sales of the product are exceeding expectations, Chief Executive Officer Tetsuo Iimori said. "There's no question that this is going to be a pillar of our retail trust business," he said in an interview. "The potential here is clearly enormous."

Japan's biggest banks are expanding their wealth management business to make up for declining profitability from lending. With most of the population's financial assets held by people over 60, there is a clear incentive to boost services for older clients.