We investigated the radioactive cesium concentrations in Barn Swallow Hirundo rustica nests that were collected from large areas of Japan in 2011 in order to understand the pattern of radioactive cesium contamination emitted by the Fukushima Daiichi Nuclear Power Plant accident. We collected 197 nests from 21 prefectures and measured radioactive cesium (Cs-134 and Cs-137) levels in 182 nests. Radioactive cesium contamination of nests was found in 13 prefectures. Cesium concentrations (Cs-134+Cs-137) in nests ranged from 33 to 90,000 Bq/kg in the Fukushima area, and the average concentration level in Fukushima was higher than in the other areas. There was a significantly positive relationship between radioactive cesium concentrations in nests and in the soil around the each nest site.