Police in Japan said Tuesday they have referred to prosecutors ramen noodle chain Ichiran, its president and six officials for allegedly making foreign students work illegally at its Osaka stores in violation of the immigration law.

Manabu Yoshitomi, 53, president of the Fukuoka-based chain serving thin noodles and pork-bone soup, and the six officials are suspected to have made Vietnamese and other foreign students work over the permitted limit of 28 hours a week at two of its stores in Osaka from September through November.

They have admitted to the charge, with Yoshitomi quoted by police as saying, "Although I was aware of the illegal practice, I had left the matter to the head office's labor management department. I am responsible for not thoroughly complying" with the law.

The managers of the two stores were among the six officials. One of the managers said the illegal practice was "necessary to continue the service."

The popular ramen chain, which was known among foreign students to offer high wages, had many foreign workers at the stores in the busy Dotonbori shopping district in Osaka.

In November, the police arrested a Vietnamese worker at one of the stores for immigration law violation and searched the head office. More foreign workers at Ichiran were subsequently arrested on the same charge.

Ichiran, founded in 1993, is best known for partitions surrounding each seat at counter tables so individual customers can "concentrate on the ramen flavor." It currently runs about 70 stores in Japan and abroad, including the United States, Hong Kong and Taiwan, according to the company website.