The body was found in the apartment in the Paris suburb of Bussy-Saint-Georges on Friday, after the flat was sold at auction following its repossession by the bank, police and judicial sources said.
The previous owner, a former security guard of Cambodian origin, appears to have committed suicide by hanging himself with a sheet.
The man, aged around 40 when he died, had not been heard from since 2005 when he filed a labour complaint against his firing.
He had cut ties with his family years before and neighbours believed he had returned to Cambodia.
His apartment was eventually sold after he failed to make mortgage payments and to cover building charges.
A police source said the new owner discovered the body when he arrived to inspect the apartment and entered with the aid of a locksmith.
“The body was in perfect condition, as was the apartment,” the source said.
Investigators were unable to explain why the body had not decomposed.
Neighbours said they had no inkling of what lay inside the flat.
“I thought it was an abandoned apartment,” said Camille, who lives in one of the four-storey building’s 20-odd apartments.
“Bussy is a commuter suburb, people don’t see each other much,” she said.