“It’s 4 o’clock. My body is trembling … I just can’t do this. I’m gonna die. I’m so tired,” one message reads.
Tadashi Ishii, Dentsu’s president, said in a statement that it was “extremely regrettable” the company had failed to “prevent overwork by a new recruit.”
“In order to take full responsibility, I would like to resign as president at a board meeting in January,” he added.
Mr Ishii’s resignation comes as Prime Minister Shinzo Abe campaigns for sweeping reforms of Japan’s employment laws, which could include tighter overtime regulation.
It is not the first time a Dentsu employee has killed themselves due to overwork. In 2000, Japan’s highest court ruled that a 24-year-old employee hanged himself the previous year because he was suffering from “horrendous working conditions”.
It is unclear how many victims of karoshi there are each year in Japan, but it is estimated the figure is in the hundreds or even the thousands.
Among them is 24-year-old Kiyotaka Serizawa, who killed himself in July 2015 after working up to 90 hours per week at an apartment maintenance company.
Japan’s rigorous working culture – where shifts of 12 hours or longer are often considered the norm – is understood to have started in the 1970s, when wages were low.
The culture continued through the 1980s boom years and is often credited with pushing Japan to become the world’s third largest economy.
But it has also been harshly criticised for leaving workers miserable and exhausted and, in some cases, suicidal. More than 25,000 people took their own lives in Japan in 2014 – about 70 per day – which is three times the rate of suicide in Britain.