The boss of Yamaguchi-gumi, Japan's largest crime syndicate, will be imprisoned as soon as the Supreme Court rejects his appeal of a gun conviction, sources said Thursday.

The top court's decision, dated Tuesday, will finalize the six-year prison sentence on Kenichi Shinoda, who is out on bail, they said.

Shinoda, 63, was convicted of conspiring with a bodyguard who was found to be in illegal possession of a gun in 1997.

Shinoda, also known as Shinobu Tsukasa by other gangsters, became the sixth don of the gang in July, succeeding Yoshinori Watanabe.

According to the National Police Agency, the Kobe-based underworld syndicate had about 39,200 members and quasi-members as of the end of last year, accounting for 45 percent of all mobsters throughout Japan.