A Japanese father who kept his mentally ill adult son in a cage for decades received a suspended jail term on Wednesday, a court spokeswoman said.

While the court found 73-year-old Yoshitane Yamasaki guilty, the judge also criticised authorities for failing to help him care for his son, who had been violent towards his parents.

Yamasaki, from the western city of Sanda, was found guilty of confining his son, now 42, for five years until this year. His son is now in the care of local authorities.

Yamasaki has confessed to caging his son for more than two decades but prosecutors only pursued five years of the crime because of the statute of limitations, local media said.

Judge Kimikazu Murakawa sentenced him to one year and six months in prison, suspended for three years, according to a spokeswoman at the Kobe District Court.

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Defence lawyers had argued for leniency because Yamasaki sought but did not receive public assistance.

His son was reportedly forced to live inside a wooden cage - one metre (3.3 feet) high and 1.8 metres wide - built in a prefabricated hut next to the main house.

Yamasaki and his wife, who died in January, allowed their son to stay in the main building with them for about 12 hours every other day, according to the Kobe Shimbun newspaper.

The couple reportedly confined their son after violent episodes, including one in which he bit his mother.

The case became public in January, when Yamasaki consulted local officials about care for his ailing wife and again raised his son's situation.

Yamasaki said he showed the cage to local officials around 1993 to seek advice, but received no feedback, Kyodo News agency reported.

The judge, while condemning Yamasaki, said local authorities had failed to provide support.

"No empathy can be extended as confining the victim for such a long time could have been avoided," the judge said, according to the Kobe newspaper.

But, he added, "the local community had a duty to take central roles as a safety net to allow those in need of assistance to live their lives with dignity."

"And the community as a whole lacked this awareness," the judge said.