原著論文 The accident at the Fukushima Daiichi nuclear power plant occurred by the Tohoku Pacific Ocean earthquake on March 11, 2011 has become the contamination with radioactive materials into the environment. Immediately after the accident, the contamination has spread to air, soil, sea water etc. More than four years has elapsed, the influence of radiation into the living area has decreased steadily. But the environmental pollution is remained. We have measured the radioactive activities of soils such as in Fukushima, Fukushima offshore seabed, Tokyo Bay seabed, Tama River. In this study, the radionuclides in soil of Arakawa River has been measured and been compared to the results of the soil in Tama River. Cs-134, 2.06-year half-life, and Cs-137, 30.17-year half-life, have been detected in the soils of Arakawa River like those of Tama River. These are the traces of the Fukushima Daiichi nuclear power plant accident. However, I-131 which has observed in Tama River is not detected.