Police are set to refer ramen chain Ichiran and some of its managerial staff to prosecutors this week for allegedly breaking the immigration law by forcing foreign students to work illegally long hours at two stores in Osaka, investigative sources said Monday.

Ichiran, founded in 1993, serves thin noodles and pork-bone soup. It is best known for surrounding each counter seat with partitions to let diners “concentrate on the ramen flavor.”

Ichiran operates about 70 stores in Japan and abroad, including in the United States, Hong Kong and Taiwan, according to its website. Its head office is in Fukuoka.

The chain is popular with foreign students because it pays better. Its stores in the busy Osaka shopping district employ many foreign workers.

In November, a Vietnamese woman was arrested for allegedly working more hours than permitted at an Ichiran store in Osaka. After police searched the head office and the outlet, more foreign workers at Ichiran were subsequently arrested on the same basis.