Jean-Luc Josuat-Verges, 48, told police he survived by eating wood and clay.
He was found thanks to some teenagers who had been exploring the caves, which are normally closed to the public.
Mr Josuat-Verges left his home on 18 December suffering from depression and telling his wife he wanted to be alone for a time, local newspapers said. Taking a bottle of whisky he drove his jeep to an abandoned mushroom farm consisting of a vast network of underground tunnels. But in the pitch blackness he got lost and for the next 35 days he had to survive as he could.
Wrapped in plastic sheeting, he said he ate wood and clay, which may have been the remains of the mushrooms.
On Thursday the teenagers, taking advantage of a teachers’ strike, went to explore the underground galleries which are supposed to be out of bounds to the public. Seeing the man’s jeep, they alerted the police who eventually found him on Friday. He was thin, very dishevelled, but alive. It was like seeing a mummy rising from the dead, one officer told a newspaper.