Tang Xili came to Japan in 2013, hoping to earn enough in three years to build a new home for her daughter. Instead, she ended up in a labor union shelter after leaving an employer she says owes her about ¥3.5 million in unpaid wages.

The 35-year-old from Yizheng, China, says she worked long hours, six days a week, was paid less than minimum wage for her overtime, and couldn't change her employer because of the terms of her visa.

"I really regret coming to Japan," she said at the shelter in Hashima, Gifu Prefecture, where she is staying in an effort to get her back wages. "I won't recommend that my friends come here to suffer."