Caesium-137 ( 137 Cs) is a major anthropogenic radionuclide released into the environment as a result of the TEPCO Fukushima Daiichi Nuclear Reactor Station accident (occurring on March 11, 2011). Rice, being a staple food in Asian countries, including Japan, and is predominantly cultivated in paddy fields. Consequently, 137 Cs present in rice is absorbed from both soil and irrigation water, making it the most important crop for estimating internal radiation doses. In this study, over the 2018-2022 cultivation periods, flood water and pore water samples were collected biweekly from paddy fields. These samples were analyzed to measure the 137 Cs activity concentration, as well as the potassium (K + ) and ammonium (NH 4 + ) concentrations. Under anaerobic conditions, the 137 Cs + activity concentration in pore water increased markedly to reach a value 20-fold higher than that in flood water, correlating with NH 4 + concentration. However, despite the release of 137 Cs + caused by increased NH 4 + concentrations in pore water due to reduction processes, the 137 Cs + /K + ratio did not increase, which was attributed to the simultaneous release of K + . The competition between 137 Cs + and K + uptake by plants indicates that rice uptake of 137 Cs is not necessarily enhanced during the waterlogging period.