A plant-operation-schedule optimization system includes a data inputting device transmitting a future operation planned value of at least one plant to be controlled; an operation-data inputting device transmitting operation data concerning the plant; and an operation-schedule calculating unit receiving the operation planned value and the operation data concerning the plant from the data inputting device and the operation data inputting device, respectively, to calculate an optimized operation schedule of the plant. The operation-schedule calculating unit includes an optimization calculator calculating the optimized operation schedule; and a database storing the kind of an operation mode, an operation pattern, and an operation cost according to the operation mode of the plant. The data stored in the database is referred to in the calculation of the optimized operation schedule in the optimization calculator.