He was thought to be the oldest man in Tokyo - but when officials went to congratulate Sogen Kato on his 111th birthday, they uncovered mummified skeletal remains lying in his bed.
Mr Kato may have been dead for 30 years according to Japanese authorities.
They grew suspicious when they went to honour Mr Kato at his address in Adachi ward, but his granddaughter told them he "doesn't want to see anybody".
Police are now investigating the family on possible fraud charges.
Welfare officials had tried to meet Mr Kato since early this year. But when they went to visit, family members repeatedly chased them away, according to Tomoko Iwamatsu, an Adachi ward official.
Authorities grew suspicious and sought an investigation by police, who forced their way into the house on Wednesday.
They discovered a mummified body, believed to be Kato, lying in his bed, wearing underwear and pyjamas, covered with a blanket.
Mr Kato's relatives told police that he had "confined himself in his room more than 30 years ago and became a living Buddha," according to a report by Jiji Press.
But the family had received 9.5 million yen ($109,000: £70,000) in widower's pension payments via Mr Kato's bank account since his wife died six years ago, and some of the money had recently been withdrawn.
The pension fund had long been unable to contact Mr Kato.
"His family must have known he has been dead all these years and acted as if nothing happened. It's so eerie," said Yutaka Muroi, a Tokyo metropolitan welfare official.