The Financial Services Agency, which falls under Aso's ministry, is investigating Mizuho Financial Group and other lenders following disclosures that gang affiliates were eluding checks meant to freeze them out of the financial system.
Mizuho's president Yasuhiro Sato and industry, government and police officials acknowledged they were struggling to comply with longstanding laws against doing business with organized crime.
Kazuhiro Omori, head of the Japan Consumer Credit Association, said his group was revising its rules and members are preparing to close any accounts violating them.
"It is very difficult. There are many cases to be reviewed," he said.
Lawmakers chided Mizuho for delegating "know your customer" responsibilities to the bank's consumer credit affiliate, Orient Corp., which was responsible for most of the 200 million yen ($2 million) of loans extended by Mizuho to yakuza fronts. But Sato told the hearing that Mizuho itself had also issued problem loans.
"It's not enough just to make rules if you don't actually carry them out," said Yuzuru Takeuchi, a lawmaker with the New Komeito Party, the coalition partner of the ruling Liberal Democrats.
Japanese securities companies have long had access to police data on gang members and gang affiliates, but sharing of that information with banks has lagged, police and other officials said.
Banks now are scaling back on lending through "tie-up" loans via the consumer credit companies that lack access to databases identifying customers with potential yakuza links.
They are also stepping up information gathering for their own databases. The financial newspaper Nihon Keizai Shimbun reported that Mizuho had collected data on up to a million potentially suspect transactions, while the other major banks had also identified tens of thousands of such cases.
Aso urged the banks to share such information with their credit affiliates.
"It's clear that the problem arose because the banks were leaving such checks up to the consumer credit companies," he said.
The scandal over mob loans has highlighted widespread problems with corporate governance in Japan due in part to the common practice of having industry officials move into regulatory jobs.
Japan has been a leading member of the Financial Action Task Force, which combats terrorism financing other illicit transactions. In its most recent assessment, the FATF noted that Japan's system for catching such transactions appeared "not to be fully and effectively implemented" and prosecutions were low compared with the scale of the country's organized crime problems.
At Wednesday's hearing, lawmakers accused officials of being too "soft" on Mizuho and sought details on possible further penalties.
Sato forfeited six month's pay over Mizuho's mob loans, and the chairman of Mizuho's banking business resigned.
"This case was very regrettable, I must say," Sato said. "I intend to work to ensure a clear break from such anti-social forces. This is my responsibility."
Asked if that was penalty enough, Sato acknowledged some people were calling for harsher penalties, but noted that his bank was not the only lender to have been caught extending mob-linked loans.
"Our handling of this was inadequate, but such loans are among the risks banks face as they provide credit and keep money circulating in the economy," he said.
