Radioactive caesium has been detected above the safety level in rice for the first time in Japan since the nuclear crisis began at the Fukushima plant.
The sample came from a Fukushima city farm about 60km from the plant.
The government is considering banning shipments from the area it was found.
There have been a series of scares over radiation in food in Japan in recent months - in beef, mushrooms and green tea among other products - but never before in the country's staple, rice.
Now caesium in concentrations above the official safety limit has been detected in a sample from a farm in Fukushima city.
The rice was being prepared for market, but Chief Cabinet Secretary Osamu Fujimura said none had been sold.
The discovery highlights the difficulty of tracking the radiation which has been spread across eastern Japan by wind and rain.
Local governments in rural areas have set up testing centres to try to ensure contaminated products do not get into the food chain. more