Okinawa Prefecture, located about 17-hundred kilometers from the Fukushima Number 1 nuclear power plant, is the latest area to confirm radioactive substances in Japan.
The nation's science ministry says the radioactive substances came from the stricken nuclear plant because of the presence of cesium-134, which has a short half-life of two years.
Before the accident, cesium-137, which has a longer half-life of 30 years, had been detected from time to time in atmospheric nuclear tests.
But those densities mostly stayed below 1 becquerel per square meter, while cesium-134, with a shorter half-life, was rarely detected.
The peak value in all Japanese prefectures was found in Ibaraki Prefecture, where levels were 970-thousand times larger than the cumulative fallout density recorded in 2009.