In 2011, the radiocesium concentration of brown rice grown (Oryza sativa L.) in mountainous areas with irrigation water that flowed from a nearby mountain forest, in Fukushima Prefecture, Japan, exceeded 500 Bq kg–1. Migration of radiocesium from contaminated irrigation water was suspected. Here, we investigated the migration of dissolved radiocesium (137Cs) from irrigation water into brown rice by determining the concentration of 137Cs in brown rice grown in pots irrigated with water containing dissolved 137Cs (0.1, 1.0 or 10 Bq L–1 137Cs). The treatment water was prepared by extracting 137Cs from conifer branches collected from the contaminated area and diluting the stock solution with tap water. We used two Gray Lowland soils (soils A and B) collected from two paddy fields in Fukushima Prefecture. Rice plants (Oryza sativa L. cv. Hitomebore) were grown in pots and treatment water was kept at a depth of 3–5 cm throughout the growing season. We calculated the amount of 137Cs absorbed by the plants by subtracting 137Cs concentrations in brown rice grown with tap water from that in brown rice grown with treatment water. The concentration of 137Cs in brown rice increased in proportion to that in the treatment water. The concentration of 137Cs in brown rice grown with water containing 10 Bq L–1 137Cs in soil A was 4.9 times that in brown rice grown in soil B, but the concentration of 137Cs in brown rice grown in either soil was not significantly different when grown with water containing 0.1 Bq L–1 137Cs. Before plants were grown, the exchangeable potassium in soil B was 3.8 times that in soil A. At least, even with irrigation water containing about 0.1 Bq L–1 of radiocesium, the migration of radiocesium into brown rice is negligible for the provisional regulation value for radiocesium in brown rice under the condition of our experiment.