Paddy Field Dam (PFD) is an attractive option for using paddy fields as green infrastructure for preventing inland floods. The runoff control devices used in PFD, which typically consist of 3-5 cm notch set devices in the outlet weir plate, can be manufactured for 10-50 USD per unit; they are thus both inexpensive and highly efficient, as they rely on the water storage function of rice paddies already present in the watershed. However, for such efforts to spread, the effectiveness and safety of PFD must be evaluated, and the understanding of residents and farmers of PFD must be promoted. In this study, a survey of drainage flow during heavy rainfall was conducted in a group of rice paddies along the Tomikawa drainage channel located in Fuchu-machi, Toyama City. The study area is approximately 8.8 ha in size, and rainfall and water levels in the drainage channel were continuously observed to obtain a rainfall-runoff relationship. The observation period was from May to November 2022, and the measurement interval was 10 minutes. During the observation period, relatively intense rainfall events on 23^rd July (40.5 mm) and 1^st Sept. (54 mm) were analyzed. The flow rates for the two rainfall events were simulated by using the paddy and soil water balance model, as well as the runoff flow when the PFD was implemented. Assuming the implementation of PFD with a 5 cm notch set device in the weir plate to control runoff, a peak cutoff effect of 42.9% was expected for the 23^rd Jul. event and 53.4% for the 1^st Sept. event. By using the model, the paddy water level was simulated under conditions of a historical extreme rainfall event on 5^th July 2018, and a safe design of notch size in the weir plate, dependent on the paddy field size, was proposed.