An in situ model paddy field experiment was conducted using non-contaminated soil to identify factors that locally increase the radiocesium activity concentrations in rice near the water inlet of paddy fields. The results demonstrated that irrigation water intake reduced exchangeable potassium concentration and increased radiocesium activity concentration in the soil near the water inlet; thus, the radiocesium activity concentration in rice was negatively correlated with the former and positively with the latter. Exchangeable potassium flushing and deposition of suspended solids in the soil owing to irrigation water intake facilitated the transfer of radiocesium to rice plants near the water inlet.