Police arrested the 18-year-old on late Tuesday immediately after he pushed Kuniaki Kariya, 38, off the platform in front of an arriving train in the western city of Okayama.

The train ran over Kariya, a local government employee, who died five hours later, police said.

Under questioning, the teenager openly admitted he pushed the man off, police said.

'If I killed someone, then I can go to jail. It didn't matter whom I'd kill,' a police spokesman quoted him as telling. The 18-year-old's name is being withheld because he is a minor under Japanese law.

The teenager was also carrying a knife with a 12-centimetre blade, police said.

'He told police that he had thought about stabbing someone,' the spokesman said. 'He said he wandered around the station but couldn't get decisive.'

The killing took place two days after a man went on a stabbing spree outside a shopping mall north of Tokyo, killing one person and seriously injuring seven more.

Masahiro Kanagawa, 24, promptly turned himself in, calling police from a nearby phone booth and saying: 'I'm the criminal. Arrest me as soon as possible.'

Kanagawa had been on the police's most-wanted list over another stabbing and killing of a stranger last week.

In a separate case, a woman in the central city of Nagoya showed up at a hospital on Monday with a fruit knife sticking out of her back, saying she had been attacked by a stranger who sped away on a bicycle. -- AFP