[en] The concentrations of radioactive cesium in the body of pond smelt, which were captured at the central part of Lake Kasumigaura from 2012 to 2015, were investigated. According to the results, contamination with radioactive cesium in Lake Kasumigaura, which occurred on March 11, 2011, increased at the highest in 2013 and decreased in 2014 and 2015. However, this decline trend was gradual and the pollution seems to be prolonged. A comparison of ¹³⁴Cs and ¹³⁷Cs concentrations showed that the former decreased rapidly, the latter hardly decreased in the five-year period because of the difference of half-life in radioactivity. Judging from this result, the present state of the pollution is expected to continue in the long-term future and there seems to be no problem on human health effect. (author)