It's the latest decision against overwork in Japan, where stoic acceptance of extended overtime has long been the norm.
The man who died was 45 years old. His wife's lawyer says the man had been under severe pressure as the lead engineer in developing a hybrid version of Toyota's Camry line.
The lawyer says in the two months up to his death, the engineer averaged more than 80 hours of overtime per month.
The ruling will allow his family to collect benefits from his work insurance.
The ruling is the most recent in a string of decisions against long working hours in Japan, which is struggling to cut down on deaths from overworking, known as "karoshi." Such deaths have steadily increased since the Health Ministry first recognized the phenomenon in 1987.
