A would-be Japanese bank robber asked staff how he should carry out the crime before meekly obeying a request to leave and then accidentally stabbing himself in the leg with a knife he was carrying.

The 58-year-old unemployed man went into a branch of the Saitama Resona Bank in the town of Kumagaya, north of Tokyo, on Wednesday, intending to rob it, a police spokesman said.

According to local media reports the man first asked a bank teller, "Any idea how you rob a bank?" The teller alerted another member of staff, who asked the man to leave.

"He left quietly when asked to," the police spokesman said.

However, the staff member escorting the man out of the bank noticed the knife sticking out of his pocket and a bloodstain on his trousers.

Police arrested the man for illegal possession of a weapon.

"He didn't brandish the knife at anyone ... but he injured himself in the leg," the police spokesman said.