The health ministry has questioned new Dentsu President Toshihiro Yamamoto on a voluntary basis about allegations the advertising giant made employees at regional offices work illegally long hours, sources said.

Dentsu Inc. has been under fire since the Christmas Day suicide of a new 24-year-old female employee was officially termed a case of karoshi (death by overwork). The incident prompted then-President Tadashi Ishii to step down.

On Dec. 28, the Health, Labor and Welfare Ministry referred the advertiser and one of its male executives to prosecutors on suspicion of forcing Matsuri Takahashi, who committed suicide in December 2015, to underreport her working hours.

The ministry had raided Dentsu branches in Nagoya, Osaka and Kyoto that November for evidence that it had broken the Labor Standards Law.

The ministry is expected to send its reports on Dentsu and the branch officials to prosecutors later this month and bring the entire investigation to an end, according to the sources.

Yamamoto, who became president in January, was apparently questioned on Thursday about whether the company was aware of illegal conduct and its labor management, as part of the procedure for building the cases, the sources said.

The government has stepped up efforts to combat excessive working hours after labor authorities determined last September that Takahashi died from overwork.