Monex Group Inc.'s online brokerage sold out ¥500 million ($4.6 million) of SoftBank Group Corp. bonds to individual Japanese investors in less than three minutes earlier this week, a sign of strong demand for the conglomerate's debt despite skepticism about some of the company's recent investments.

Tokyo-based Monex started selling the bonds at 9 a.m. on Monday and ran out two minutes and 26 seconds later, said Shinichiro Yamada, general manager in the firm's products department. That's despite Typhoon Faxai's impact on trains, airplane schedules and other infrastructure in the greater Tokyo area.

It takes the brokerage about two weeks to sell out on average.