Some 38.2 percent of employees who worked overtime in September were not paid for their extra hours on the job, a survey by the Research Institute for the Advancement of Living Standards has shown.
The figure is the highest rate since the current survey method was adopted in 2011.
"Just like in the overwork-related suicide of the new Dentsu Inc. employee (in December 2015), there are a lot of cases where employees find it hard to refuse to do free overtime. It's a serious problem," commented the institute's Deputy Director General Shigeru Oshima.
The institute gave online questionnaires to 2,000 workers across Japan from Oct. 1 to 6, of whom 38.5 percent said they had worked overtime in September, averaging 40.3 hours for the month.
Of those, 38.2 percent replied that they had done unpaid overtime. Two-thirds of these workers said they had decided to underreport it, and roughly 20 percent said their bosses had ordered them to underreport it, with the unpaid extra time averaging 17.6 hours.
Among permanent employees, 53 percent of men reported doing overtime, which averaged 49.4 hours -- five hours more than the figure for September 2015 and significantly beyond the legal limit of 45 hours. Some 34.7 percent of women said they had worked overtime, averaging 26.4 hours.
About 6.5 percent of men and 1.8 percent of women had surpassed the so-called "death from overwork danger line" of 80 overtime hours in September.
Among all workers who did unpaid overtime, 30.1 percent said they didn't report the extra hours because "the workplace atmosphere makes it difficult," while some 25.6 percent replied that their employers "impose overtime pay limits."