The man is believed to have burned to death in the outhouse belonging to his summer cottage near the village of Björklinge. Police believe he was paying an early morning toilet visit when the fire broke out. His partner called the fire brigade.
Police say they suspect that the fire was started deliberately, and that it could be a case of murder.
Police forensic scientists have saved evidence following an examination of the scene. Police are due to carry out a post-mortem on the dead man, who has not been positively identified.