People have reported vomiting, diarrhoea and other symptoms of food poisoning after eating products including pizza and lasagne made by a subsidiary of Maruha Nichiro Holdings, the nation's largest seafood firm.
The number of people affected by the tainted food has now risen to over 1,000, with more than 200 taken ill in the northern main island of Hokkaido alone, Jiji said.
In western Osaka prefecture, a nine-month-old baby was hospitalised with vomiting on Monday after eating a product called creamy corn croquettes, the report said.
Detectives are looking at the possibility that the pesticide was deliberately added to the food at some stage of production at a factory in Gunma, north of Tokyo, Jiji said.
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The food maker has recalled 6.4 million potentially tainted products, with 1.49 million packages recovered so far, she said. None of the products in question had been shipped overseas, the company said.
However, a much-vaunted reputation for safe and high quality food has been badly affected by the Fukushima atomic disaster, which saw acres of farmland polluted by nuclear fall-out and many countries restrict agricultural imports from the area.