Photo/IllutrationAdvertising giant Dentsu Inc.’s headquarters building in Tokyo’s Minato Ward (Yasuhiro Sugimoto)

Labor authorities on Dec. 28 sent papers to prosecutors against advertising giant Dentsu Inc. and a senior official on suspicion of violating the Labor Standards Law by having an employee work illegally long hours, which drove her to suicide.

The official who was referred is the former boss of Matsuri Takahashi, the 24-year-old employee who took her life in a company dormitory in the capital in December 2015.

The Tokyo-based Mita Labor Standard Inspection Office of the Ministry of Health, Labor and Welfare ruled in late September that Takahashi's death was due to overwork. It acknowledged that she worked 105 hours of overtime from Oct. 9 to Nov. 7.

Takahashi, who joined the company in April 2015, worked under the official in the section handling online ads.

The official is believed to have had Takahashi underreport her overtime hours after having her work beyond the ceiling agreed upon by the company management and the labor union, according to investigative sources.

It is also alleged that at least 30 employees at the company’s main office in Tokyo as well as three branches--Kansai, Chubu and Kyoto--reported fewer work hours than they actually put in.

The ministry intends to uncover the entire scope of Dentsu’s workplace practices, suspecting that working illegally long hours is a common occurrence.

It also expects to investigate the company’s nonpayment of overtime for employees.

The ministry is seeking to establish a case against about 10 senior officials for now, including an executive who was in charge of labor management. If it comes up with evidence of illegal practices, it will send additional papers to prosecutors.

After the ruling on Takahashi’s death made headlines across the nation and generated public outrage, the ministry moved swiftly to raid Dentsu's headquarters in November for evidence.

The ministry’s investigators questioned Tadashi Ishii, president of Dentsu, and other directors on a voluntary basis over three days from Dec. 23 to determine if they can pursue the case.

(This article was written by Takuro Chiba and Tatsuro Kawai.)