Dentsu Inc. President Tadashi Ishii said he will step down in January to take responsibility for the overwork-related suicide of a 24-year-old employee at the advertising agency.

"We deeply regret failing to prevent the overwork of our new recruit. I offer my sincere apology," Ishii, 65, said at a news conference Wednesday evening in Tokyo.

The news conference was held after the labor ministry referred Dentsu and one of its executives to prosecutors the same day on suspicion of forcing Matsuri Takahashi, who joined the company in April 2015, to work and underreport illegally long hours, leading to her suicide on Christmas Day last year.