The health ministry has reported that 36.1 kg of porcini mushrooms imported from Italy through Narita airport where found to emit levels of cesium radiation above the government-set level.
Health, Labor and Welfare Ministry officials told the importer to return the mushrooms to the exporter.
Radiation from the former Soviet nuclear plant at Chernobyl, which suffered one of the worst nuclear accidents on record in 1986, may have polluted the mushrooms, officials said.
While excessive radiation levels were frequently detected in imported food products in the 1980s, there have been no reports of detection in recent years.
The last such incident occurred in January 1998 when excessive radiation was detected in dried porcini mushrooms also from Italy.
Following the accident at Chernobyl, the government set the maximum level of cesium 134 and cesium 137 radiation at 370 becquerels per kg for food products imported from the former Soviet Union and Europe.
The radiation in the latest cargo of mushrooms from Italy measured 418 becquerels, ministry officials said.
Porcini mushrooms are used in Italian cooking for pasta and sauteed dishes. Given that radioactive materials can easily accumulate within mushrooms, imports of mushrooms from all countries suspected of being exposed to nuclear pollution are subject to inspection.