Ishii said in a press conference he felt “deep responsibility as a person for overseeing the management of the company”.
Matsuri Takahashi, 24, jumped to her death from a company dormitory on Christmas Day 2015 after telling friends on Twitter of being forced to work extreme hours.
It later emerged she had worked more than 100 hours overtime in a single month.
According to The Japan Times, Ishii told a Tokyo press conference on Wednesday: “We deeply regret failing to prevent the overwork of our new recruit. I offer my sincere apology.”
The press conference was held after the labor ministry referred Dentsu to prosecutors on suspicion of forcing Takahashi, who joined the company in April 2015, to work and underreport illegally long hours.
Shortly before ending her life, Takahashi told friends on twitter: “They’re making me work Saturdays and Sundays again. I seriously want to end it all.”
A Tokyo labour department ruling in October concluded that she had suffered mental collapse due to the burden of overwork.
The Tokyo labor standards inspection office also recognised her suicide as a case of karoshi, the Japanese word for death from overwork.
Dentsu issued a brief formal announcement yesterday, confirming Ishii will tender his resignation at the end of Dentsu board meeting next month
“A successor Representative Director and President & CEO will be appointed at the…..board meeting and then announced immediately after the meeting,” Dentsu said.
The announcement made no mention of the company being referred to Japanese prosecutors, or of the death of Takahashi.
Takahashi’s death came three years after a young creative in Indonesia died allegedly as a result of overwork, and an account executive in China, also 24 years of age, passed over six months prior after suffering from a stress-related heart attack after working long hours continually for a month.