Monthly fieldwork was conducted in the paddies of Fukushima Prefecture in 2016, to obtain samples of the paddy soil and rice plants. The monthly change in the radioactivity concentrations of 137 Cs, 134 Cs, and 40 K of the samples was investigated, using a germanium semiconductor detector. Three-phase transfer factors (TFs) of 137 Cs from the paddy soil to the roots (TF 1 ), from the roots to the leaves (TF 2 ), and from the leaves to the ears (TF 3 ) of rice plants were calculated. The results

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... showed that the radioactivity concentration of 137 Cs and 134 Cs in the paddy soil varied seasonally, while the concentration of 40 K showed an almost opposite seasonal change compared to 137 Cs and 134 Cs. The radioactivity concentration of 137 Cs and 134 Cs in the roots increased 60 days after planting, while the concentration of 40 K decreased. Furthermore, the radioactivity concentration of 137 Cs, 134 Cs, and 40 K in the leaves and ears decreased over time. Correlations of the TFs of 137 Cs with the radioactivity concentration of 40 K suggested that 137 Cs and 40 K were competitively absorbed by the roots, however, they were transported to the leaves and ears in the same manner. In conclusion, the transportation of 137 Cs (TF 2 and TF 3 ) in rice plants was high despite the low absorption of 137 Cs (TF 1 ) in the early stage of rice growth. Therefore, it is recommended that the potassium concentration in the paddy should be high during the early stage of growth to prevent radioactive cesium contamination.