Officials wanted to honor Sogen Kato, Tokyo's oldest person on his 111th birthday. When they went to his house in Adachi ward, his granddaughter told them that he didn't want to see anybody.
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, where they discovered mummified skeletal remains lying in his bed.
Mr Kato may have been dead for 30 years according to Japanese authorities.
The mummified body, believed to be Kato,was 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) 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pension payments via Mr Kato's bank account since his wife died six years ago, and some of the money had recently been withdrawn.
Police are now investigating the family on possible fraud charges.