JAPAN (BLOOMBERG) - Dentsu President Tadashi Ishii said he would step down in January to take responsibility for the suicide of an employee who had worked excessive hours.

Mr Ishii's move comes after the Tokyo Labour Bureau said on Wednesday (Dec 28) that it would refer the company and officials who manage working hours to prosecutors. The 65-year-old has been president since 2011.

The world's fifth-largest advertising agency holding group - and Japan's biggest - said in November that it was cooperating with a Labour Ministry investigation of its working conditions following the suicide of a 24-year-old employee on Christmas Day last year. Since last month, workers have been barred from logging more than 65 hours of overtime a month, down from the previous limit of 70.

Ms Matsuri Takahashi's suicide put Dentsu under the spotlight over excessive hours clocked by the company's employees. She had worked 105 hours of overtime over the course of a month from Oct 9, 2015, according to a report in the Asahi newspaper.

Work issues were a contributing factor in 2,159 suicides in 2015, according to a health and labour ministry White Paper published in October. The paper cited data from the National Police Agency and Cabinet Office.

Dentsu is especially dominant in controlling the flow of advertising into major domestic television networks and newspapers.

The shares fell as much as 1.6 per cent in Tokyo and traded 1.1 per cent lower at 5,480 yen as of 9.08am local time. The company has a market value of US$13.5 billion (S$19.6 billion).

The agency's clients have included LVMH Moet Hennessy Louis Vuitton, Diageo, Nestle, SoftBank Group and Electronic Arts, according to data compiled by Bloomberg. It had about 43,000 employees in 124 countries worldwide, with about 49 per cent in Japan, according to its annual report.