Japan's Mitsubishi Electric was yesterday referred to prosecutors for allegedly forcing excessive overtime on a male employee, who was later diagnosed with depression and then sacked.

The unnamed 31-year-old, who joined the firm in April 2013, is said to have harboured suicidal thoughts after allegedly being made to clock more than 160 hours of overtime in one month and putting up with a torrent of abuse from his supervisor.

The PhD holder also said he was not granted any days off for more than a month, and was made to officially report just 59 hours and 30 minutes of overtime between Jan 16 and Feb 15 in 2014 - just under the 60-hour ceiling agreed upon by company management and the labour union.

But a labour ministry probe into the same period has so far managed to corroborate about 78 hours of overtime, prompting it to take action.

His supervisor, who was also referred to prosecutors, had allegedly taunted him with insults such as "How could you complete your doctorate?" and "Even a junior high school student can do your job".

The latest case comes a mere fortnight after Japan's largest advertising agency Dentsu and one of its executives were likewise implicated for allegedly violating labour laws, prompting the resignation of president Tadashi Ishii.

In that case, Ms Matsuri Takahashi, 24, killed herself on Christmas Day in 2015. She had been made to work 105 hours of overtime in October 2015, and fell into depression the following month.

A public relations officer at Mitsubishi Electric said yesterday that the company "will sincerely deal with the matter" and also take steps to "thoroughly implement proper management of working hours".

The employee was in charge of research and development of lasers used in household appliances at the firm's Kamakura centre.

He took a leave of absence in June 2014 after being diagnosed with depression, and was dismissed last June when he failed to report to work after the company-mandated treatment period.

"No one seems to think there is something wrong with a corporate culture that pushes employees to their limits to achieve results," the man reportedly said.

Japan's first White Paper on karoshi, or death from overwork, published in October last year showed that work issues had led to 2,159 suicides in Japan in 2015.

Prime Minister Shinzo Abe last week urged business leaders to push for reforms in working conditions to eradicate a culture of long hours.

Walter Sim