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Japanese police have arrested a 73-year-old man suspected of keeping his son in a wooden cage for over 20 years.

Yoshitane Yamasaki said he locked up his son, now 42, because he suffered mental problems and was sometimes violent, according to officials quoted by public broadcaster NHK.

The cage, 1m in height and less than 2m wide, was in a hut next to Mr Yamasaki's house in the city of Sanda.

The son, who now suffers a bent back, is being looked after by authorities.

Local media report he has been in the care of a social welfare facility in western Japan since January.

A visit by a city official to Mr Yamasaki's home at the time alerted authorities to the 42-year-old man's confinement.

Investigators believe Mr Yamasaki began confining his mentally ill son from the age of 16, when he began to "act up".

For the time being, police have only arrested the pensioner for caging his son for 36 hours on 18 January.

Mr Yamasaki has reportedly admitted the allegations, telling authorities that he fed his son every day, and allowed him to bathe every other day.