TOKYO - An outlet of popular ramen chain Ichiran was on Wednesday (Nov 29) raided by Japanese police on suspicions that it was illegally hiring a Vietnamese worker.

Police searched a shop at the Dotonbori downtown entertainment district in Osaka, as well as the chain's main headquarters in Fukuoka on the southwestern island of Kyushu.

Their raids came one day after Osaka police nabbed a 29-year-old Vietnamese woman for illegally working at the restaurant chain without the necessary work permits.

Ichiran has gained immense popularity among visitors from Singapore and around the world for its brand of tonkatsu pork-based broth and thin Hakata-style ramen. Its unique selling point is its concept of partitioned seats in-store, which it dubs as private "flavour-concentration booths" to promote solo dining.

Police said that the unnamed woman had come to Japan as an international student at a vocational school. She began working at the Dotonbori store as a part-time employee in July last year.

But she was expelled from her school in March this year, police said, which meant she would also have lost her rights to continue being employed.

When a business operator hires a foreigner, Japanese laws mandate that the status of their residence as well as length of stay be notified to labour authorities. While Ichiran has been doing so, police have found that the company had failed to report this in the case of the arrested Vietnamese employee.

A company spokesman was quoted by public broadcaster NHK as saying on Wednesday that the woman did not declare to the firm that she has been expelled from school.

"This was a fact that we were unable to independently find out on our own. That said, we will work to strengthen our system of checks."

The chain, which was founded in 1993 in Fukuoka, now has 69 outlets throughout Japan and four abroad - two in Hong Kong and one each in Taiwan and New York.

Of the 6,300 part-time employees working across the 69 Japan stores, about 1,200 are foreigners, NHK reported.