The head of the Keidanren business lobby has said that corporate executives should take responsibility for preventing harm to employees from issues related to excessive work hours.

"Top company (executives) must lead efforts to prevent deaths from overwork," Sadayuki Sakakibara said Monday at a news conference after labor authorities recently ruled that a Dentsu Inc. employee's suicide in December was due to overwork.

The tragedy led to a probe of the advertising agency and its work practices.

It also became public last week that Dentsu was directed by authorities in August 2015, before the woman's death, to ensure its employees were not working unlawfully long hours.

Referring to the death of the 24-year-old woman, Matsuri Takahashi, who killed herself after having worked over 100 overtime hours a month in fall 2015, Sakakibara said: "It is an extremely regrettable case. It should never happen."

Before her suicide, the Dentsu employee left a number of comments on social media referring to her harsh working conditions and fatigue, including one stating "my body and heart are totally drained."

In an attempt to reduce overtime hours, Dentsu turned off all lights at 10:05 p.m. Monday. The company plans to continue to do so for the time being at its headquarters in Tokyo and all its branches including the one in Osaka.

"To maintain the health of employees and comply with regulations, we will make every effort to improve labor conditions. As part of it, we have decided to turn off all the office lights and ask employees to leave the office by 10 p.m. from today," Dentsu said in a statement.