A Japanese court has ordered a restaurant chain and two personnel to pay more than half a million dollars damages to the family of a man who killed himself after being forced to work nearly 200 hours overtime a month.

Tokyo District Court said the president of Tokyo-based Sun Challenge, a steak house chain, and another official had been culpable in failing to stop the unidentified employee from working excessive hours.

"With only one holiday given to him every several months, the psychological load of prolonged work and power harassment caused his mental disorder," said presiding judge Akira Yamada, according to a Kyodo News report on Tuesday.

Yamada ordered the company and its two officials to pay a total of 58 million yen ($510,000) to the parents of the man, who was 24 when he took his own life in November 2010.

The employee began working for Sun Challenge in 2007 and was appointed restaurant manager in July 2009.

In the seven months before he hanged himself, he had worked an average of 190 hours overtime every month and had taken just two days off.

He had also been subjected to physical violence and verbal attacks by his supervisor.

The ruling was "epoch-making", a lawyer for the man's parents told Kyodo, noting that unusually in a suicide claim, there had been no finding of comparable negligence on the part of the employee to offset the blame attached to the company.

"This is a ruling that encourages workers suffering from prolonged work and power harassment," the lawyer, who was not named, was quoted as saying.

Japan's culture of long working hours and unpaid overtime is regularly criticised as a leading cause of mental and physical illness among employees.

The term "karoshi", which means "death by overwork", entered the lexicon a few years ago amid a surge in the number of people dying because of stress-related problems, or taking their own lives.