To prevent the main steam temperature difference from enlarging by calculating the upper and lower limit values, based on the steam temperature differences of each boiler and altering a load distribution command value within the limit values, based on the upper and lower limit values and a load distribution altering value, thereby altering the load distribution without varying the overall generated amount.

A setter 22 sets a value, obtained by subtracting the load of No.2 gas turbine from the load of No.1 gas turbine, as a load distribution altering value 33. A corrective function generator 18 sets the upper and lower limit values 36 of load distribution for each gas turbine. A load distribution command value is altered within the upper and lower limit values. According to the method, the load distribution can be altered according to the characteristics of each gas turbine generator, and the temperature difference can be prevented from enlarging as well.