Just four months before the December work-related suicide of a young employee last year, labor authorities had instructed advertising giant Dentsu Inc. to correct its staff's unlawfully long working hours, company officials said Wednesday.

The finding shows that despite the warning, Dentsu failed to take effective measures to redress working conditions that led Matsuri Takahashi, 24, to work over 100 hours a month in overtime in the fall of 2015.

Takahashi killed herself on Dec. 25 of that year, and her death was recognized last month by the Tokyo Labor Bureau's Mita office as having been work-related.