The purpose of this study is to construct a monthly water balance model using a tank distributed to each grid in a drainage basin. The study area is the Sorachi river basin (2531 km2) in Hokkaido. The grid size is 1km × 1km, which is common to the monthly climate grid data (Japan Meteorological Agency). The grid tank model is composed of a main model and a sub model for snow accumulation and snowmelt. The sub model is used for two purposes to separate snowfall from total precipitation in the original data and to surmise snowmelt, as the input to the main model. The monthly temperature was used for these purposes. Total runoff from the main tank is the sum of three components (surface, interflow and baseflow runoff) from separated three outlets, which are defined by four parameters; heights of two outlets from the tank bottom, and two runoff ratios to water storage depth to the outlet and the bottom. The optimized values of the parameters were obtained by minimizing difference between the monthly model output and observed monthly river discharge (Ministry of Land, Infrastructure and Transport). The data term; 1971 to 2000 is common for both the climate and the discharge used in this study. Using this model with optimized parameters we were able to simulate the intra - annual variation of water discharge from the drainage basin with sufficient accuracy. In conclusion this type of the distributed grid model is efficient and useful to simulate the seasonal changes of basin water balance.