The shield effects of floodwater on radiation from paddy field soil were investigated by field observation and model calculation. The devastating accident at Fukushima Daiichi Nuclear Power Plant at the time of Great East Japan Earthquake in March 2011 caused considerable radioactive leakage resulting in precipitation of a lot of radioactive substances on the ground in the surrounding area. It is reported that about 160 thousand people are still evacuated in February 2012 because of the strong radioactivity. The reduction of the radioactivity at the residential area is strongly required to realize earlier returning home of evacuated residents. As residential zones in the stricken area are typically surrounded by paddy fields, it is expected that flooding on the paddy fields brings about shield effects on radiation from the paddy soil. It is a quite practical method because paddy fields are originally designed to store floodwater in order to grow rice plants. Moreover, because the radioactive ray going to the surrounding homes must be emitted at a smaller angle to the ground surface, it must travel longer way through water when the floodwater covers the wide area, bringing about more reduction of radioactivity by the floodwater. In this study, the reduction of radioactivity in the air according to the increase in the floodwater depth was observed in a paddy field plot which had received considerable radioactive substances. The observation result was roughly reproduced by a simple model based on the theoretical shield effect by water.