The Japanese company says the employee, who was in a managerial position, traded stocks multiple times using information obtained at work.
The bank says the employee told the company about the transactions on Wednesday.
The bank says it has confirmed that the alleged insider trading took place after examining records submitted by the employee, who was dismissed on Friday.
The bank says it will fully cooperate with investigations by relevant authorities, and set up an investigative committee, including third-party members.
The bank has been holding training sessions twice a year to prevent insider trading. Employees are also asked to submit written oaths.
In principle, those who work in sections that deal with insider information are prohibited from trading stocks and bonds.
Company President Oyama Kazuya apologized at a news conference on Friday. He said he takes the matter seriously, and believes there was no organizational involvement in the case.
