The boss of Japan's biggest ad company Dentsu is stepping down after an overworked employee committed suicide

Tadashi Ishii, the president and CEO of Japan's largest advertising company - and the fifth biggest advertising agency holding group in the world - is stepping down from his role after one of the firm's young employees committed suicide after working excessive hours.

Ishii said in a Tokyo news conference on Wednesday he was resigning from his role to take responsibility for the work-related suicide and for failing to achieve a "dramatic reform of overwork," The Financial Times reported. He has led the compay since 2011.

Matsuri Takahashi, a 24-year-old former Dentsu employee, fell to her death from the window of her corporate dormitory in December 2015, The Japan Times reported.

Japanese Labor inspectors ruled in October this year her suicide was caused by "karoshi," a Japanese word meaning death by overwork.

Lawyers for Takahashi's family said in the month starting October 9, just nine months after she had joined the company, Takahashi had worked 105 hours of overtime and wrote emails to her mother and messages on social media outlining her difficult working life, according to The Financial Times and The Wall Street Journal.

"As a recently joined employee, she was working to her utmost but was being treated as any other employee [at the company]. I don't think that it is overstating the situation to describe it as 'power harassment'," Mr Ishii said at the press conference on Wednesday.

Dentsu did not immediately respond to a request for comment.

The Financial Times notes that the press conference came on the same day government officials filed papers with the prosecutor's office that could see criminal charges brought against the manager Takahashi was reporting into at the time of her death. Dentsu's headquarters in Tokyo were raided by the Labor Bureau in October this year, related to Takahashi's death.

Earlier this month, Dentsu announced it had updated measures across eight categories to improve its working culture. Changes included having mental health professionals on site at all time and prohibiting employees from working between 10 p.m. and 5 a.m.

The news of Ishii's resignation also comes after Dentsu admitted the digital subsidiary in its home market had overbilled 111 advertisers, amounting to some 230 million Japanese Yen ($2.29 million) in fees. Takahashi had worked in that division, The Financial Times reported.

In a December 26 update, Dentsu said it will present its internal investigation on the overbilling matter "after the end of January 2017." The company added that it has found no issues that will materially impact the company's financial results.