Japan’s biggest advertising agency has been fined 500,000 yen (£3,385) for forcing staff to work illegally long overtime, after the death of one of its employees forced the country to confront its unforgiving work culture.

Work practices at Dentsu came under scrutiny after it emerged last year that one of its employees had been driven to suicide by her brutal work schedule.

Matsuri Takahashi killed herself on Christmas Day in 2015 in a case that labour standards inspectors later ruled as karoshi, or death from overwork.

Takahashi, 24, was forced to work 100 hours’ overtime in the month leading up to her death. Weeks before she died, she posted on social media: “I want to die.” Another message read: “I’m physically and mentally shattered.”

While the token fine is expected to attract criticism, Takahashi’s lawyer, Hiroshi Kawahito, described the ruling as a “historic” event. “It is very meaningful that a company has been punished,” the Nikkei business newspaper quoted him as saying. “Dentsu’s crime has been confirmed.”

Takahashi’s death sparked a national outcry and forced the government to draw up guidelines that cap overtime at 100 hours a month, although critics say the limit needs to be lower to protect employees’ health.

Friday’s ruling in the Dentsu case came a day after Japan’s public broadcaster, NHK, admitted that one of its employees had died from karoshi four years ago.

Miwa Sado, a political reporter in Tokyo, had worked 159 hours of overtime in the month before she died of heart failure, NHK said.

Sado, 31, was found dead in her bed in July 2013, reportedly holding her mobile phone. “My heart breaks at the thought that she may have wanted to call me,” her mother told the Asahi Shimbun.

“With Miwa gone, I feel like half of my body has been torn away. I won’t be able to laugh for real for the rest of my life.”

NHK’s president, Ryoichi Ueda, vowed to improve work practices at the broadcaster. “We are sorry that we lost an excellent reporter and take seriously the fact that her death was recognised as work-related,” Ueda told reporters. “We will continue to work for reform in cooperation with her parents.”

A white paper on karoshi approved by Japan’s cabinet on Friday said that male employees in their 40s were at highest risk of killing themselves due to overwork.

The report said there were 191 karoshi cases in the year ending in March 2017, adding that 7.7% of employees regularly put in more than 20 hours’ overtime a week.

It said 368 people – 352 of them men – had killed themselves between January 2010 and March 2015 in officially recognised cases that involved compensation payouts. However, the actual number of karoshi-related suicides is thought to be significantly higher.

Dozens of other people die every year from heart failure, strokes and other conditions brought on by punishingly long working hours.

Prosecutors had taken action against Dentsu for forcing Takahashi and three other employees to work overtime between October and December 2015 beyond a monthly 50-hour limit agreed with the company’s union, Japanese media said.

“Illegal long working hours were becoming the norm” at Dentsu, the judge Tsutomu Kikuchi said in his ruling at the Tokyo summary court, according to Kyodo news agency. “Overtime work without payment was also rampant” at the firm, he added.