An unemployed ex-gangster in Japan in love with a 15-year-old girl has chopped off his little finger and mailed it to her father twice in an unsuccessful bid to prove his commitment, police have said.

Hiroyuki Yoshikawa, 36, was arrested on Monday after the teenager's father told police the finger had been sent to him again, after marking it return to sender the first time, a police spokesman said.

When he first sent the finger, Yoshikawa allegedly enclosed a note: "Please let us go out in exchange for this finger. I will send it again and again until you accept my request".

The father, 43, opposed the relationship and was unpersuaded by the finger.

Japanese gangsters, known as the Yakuza, commonly chop off their little fingers to atone for such defects as betrayal or defeat in turf battles.