The long-haired former head of the Aum Supreme Truth sect was sentenced to death in February and is undergoing an appeal trial, which his lawyers want suspended so he can undergo psychological tests.
Prison authorities sent a document to the Tokyo High Court revealing that the 49-year-old needed constant assistance in prison and wore diapers, the Jiji Press said.
The Asahi Shimbun newspaper quoted the report as saying Asahara, who for years has suffered partial blindness, was sometimes “muttering to himself but hardly speaks.”
But the prison authorities concluded there was “no abnormality regarding his remarks or behaviour,” the newspaper said.
The authorities said Asahara “cannot live daily life without help”, according to the Sankei Shimbun daily.
Members of the Aum Supreme Truth cult, which was founded in 1984, spread Nazi-invented sarin gas on the Tokyo subway, killing 12 people and injuring thousands.
Cult members had testified that the offences were committed under orders of Asahara, who reportedly wanted to thwart a police raid on the sect.
The cult, renamed as Aleph, has acknowledged responsibility for the crimes and apologised.