At least 30 employees at Dentsu Inc. have reported their monthly overtime work as being more than 100 hours less than they actually worked, a sign that the practice was widespread at the Japanese advertising giant, sources said Monday.

The findings follow revelations that a 24-year-old employee committed suicide due to overwork late last year. Labor authorities suspect the company may have been systematically showing employees' overtime work as shorter than the actual figures, the sources said.

The family of the employee, Matsuri Takahashi, claims she worked 105 overtime hours a month, exceeding the limit of 70 hours set in a labor-management agreement, before developing symptoms of depression.