Tokyo police and immigration officials arrested five Mongolian and Chinese workers late Monday and early Tuesday in a raid on a food-processing unit of mayonnaise maker Q.P. Corp.

The Metropolitan Police Department and the Tokyo Regional Immigration Bureau said they searched the head office and a plant of Salad Club Inc. in Fuchu, western Tokyo, and another plant in Goka, Ibaraki Prefecture, on suspicion that foreigners without proper visas were working there.

In addition to the three Chinese — two of whom are women — and two Mongolian men arrested, 21 others were being questioned, they said. The five are suspected of entering Japan illegally.

Police and immigration authorities said they were questioning Salad Club officials on suspicion that the company knowingly employed foreigners who did not have proper visas.

With its plants running round the clock, most of the workers assigned to night shifts are foreigners, they said.

Salad Club, founded in 1999, has two more plants in Hyogo and Saga prefectures. Its 328 employees include 60 foreigners, according to the company.

Speaking at a news conference Tuesday morning at Q.P.’s headquarters in Tokyo, Managing Director Sukefumi Ito apologized for the incident but stressed that the group is not intentionally hiring illegal immigrants.

The firm said it checks documents such as passports, foreigner registration cards and work permits before hiring non-Japanese.

It added that the arrests were probably the result of a failure to confirm the expiration dates of the workers’ work permits when renewing their employment contracts.

Q.P. said some 15 of its affiliates have employed about 1,000 foreigners. It reported 437 billion yen in consolidated sales in the business year to Nov. 30, 2003.