The objective of this study is the development of the diagnostic method of the storage and regulation functions of the farm pond. A simulation model for estimating the water level of the farm pond is developed. The water utilization is clarified in a farm pond command area located in Uwaba area, Saga prefecture, south-west Japan. In this area, the water utilization is complicated because the land use condition is various. The amount of the paddy and upland irrigation water in the study site is quantified to estimate the outflow rate. The inflow rate is estimated, considering the operation of the pumps. The temporal change of the water level of the farm pond is calculated using the inflow and outflow rates. The simulated water level reaches the low water level in April and August. The low water level in April is brought because the water demands for puddling and paddy planting increase. In August, the successive no-rain days cause the low water level. The tendency of the temporal change of the water level agrees with the past observational results. The water level is simulated using the data in the year with shortage of rainfall. The simulation result indicates that the shortage of the irrigation water is serious in the year with shortage of rainfall. To avoid the shortage of the irrigation water, assuming that the rotational irrigation is introduced, the water level of the farm pond is simulated. The simulation result indicates that the rotational irrigation is effective to supply the irrigation water to the total farm land in the year with shortage of rainfall.