Shoppers have reported vomiting, diarrhoea and other symptoms of food poisoning after eating food produced at a plant in Gunma, north of Tokyo, according to surveys carried out by the Asahi Shimbun and other local media.
The plant, run by a subsidiary of the nation's largest seafood firm Maruha Nichiro Holdings, is at the centre of the nation's latest food poisoning scandal.
Japanese police have launched an investigation into the company after it revealed last month that some of its frozen food products were tainted with malathion, an agricultural chemical often used to kill aphid in corn and rice fields.
Asahi Shimbun said it found the number of people who fell sick "exceeded 300", while national broadcaster NHK said yesterday that 359 people had been taken ill.
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Calls also included complaints about the unusual odour of some products and ways to return the products, the spokesman said.
Maruha Nichiro said that the products in question had not been shipped overseas.
The spokesman declined to comment on a possible impact of the incident on the company's earnings, only saying: "We have to specify the cause first."
While incidents of food poisoning have occurred in Japan, including in August 2012 when E. Coli-contaminated cabbage killed seven people and sickened dozens, food standards are relatively high.
However, the country's reputation for safe and high quality food suffered a body-blow from the after-effects of the Fukushima atomic disaster, in which acres of farmland were polluted by nuclear fall-out.