The Financial Services Agency is investigating securities houses to determine if they complied with regulations when profiting from an erroneous sale order placed by Mizuho Securities Co.

According to agency sources, the probe involves brokerage houses' trading of shares Dec. 8 of J-Com Co. on their own account, separate from those for their clients.

To forestall such serious consequences as huge losses from an erroneous trade action, legal regulations and in-house rules of many brokerages place a limit on trading in shares of one company or trading of shares by one employee.