In the studies on operations of a group of irrigation dams, the major issues regarding the optimum rules for operations are related to establishment of a method for technical evaluation on operations and objective terms for optimum operation. The present study proposes a method of simulation by a synthetic hydrologic model and an objective variable of total capacity of a group of dams. The existing irrigation scheme in Iwaki river basin is used for modeling t he structure of irrigation and drainage canal and diversion system. The results show that a method of a synthetic hydrologic model taking total capacity of a dam group into account is effective for formulating operating rules. Difference in minimum volume of the total capacity of a dam group caused by a change of operating rules on the priority of outlet discharge reaches 8% of the total water volume, which is the same magnitude with the minimum capacity of middle scale dams.