In this paper, the study area is selected Sungju Reservoir which was constructed with an agricultural purpose and determined the optimal water management plan among the five cases of classified irrigation area by using Linear Programming. As a results of reservoir operation, the additional water quantity of classified irrigation area showed 16.036×106m3/year, 19.404×106m3/year, 18.864×1006m3/year, 4.032×106m3/year and 0.672×106m3/year and the total water supply quantity showed 69.628×106m3/year, 70.048×106m3/year, 67.979×106m3/year, 67.979×106m3/year, and 69.939×106m3/year respectively. Therefore, the case-Ⅱ was adopted with water management plan of optimum. It is also known that the maximum irrigation area augmentation effect appears in the case which will use the additional water quantity in field irrigation of the case-Ⅱ which was adopted.