[en] A nuclear power plant accident, which occurred on March 11, 2011, caused severe radioactive contamination in Fukushima. We initiated an environmental radioactive survey in Minami-Soma City the following October. The city is located approximately 10-40 km north of the TEPCO's Fukushima Daiichi Nuclear Power Plant. The study reported here involves an environmental radioactive survey conducted along three rivers and their tributary streams. Water and bottom sands were collected from four to six sampling locations along each river. Water samples (1 L) were dried on thin Teflon sheets, and gamma-ray measurements were performed with a well-type Ge detector. Sand samples were homogenized after drying, and approximately 50 g were measured for gamma-rays with a low-background coaxial-type Ge detector. The Cs concentrations ("1"3"7Cs+"1"3"4Cs) were determined to be 10,000-20,000 Bq/kg in the bottom sand collected from the upper reaches of the rivers, whereas the concentrations measured from samples collected near coastal down reaches were measured at about 200 Bq/kg. The Cs concentrations measured in river water were about 0.8 Bq/L in the upper reaches of the rivers and 0.3 Bq/L in the lower reaches, indicating that the Cs concentration in water is quite low. It is necessary to study the behavior of the contaminants in the upper reaches of the river system, and determine if they will move to the down stream area hereafter. (author)