A combined power generation unit includes a gasification unit with a gasification furnace that is used to produce fuel gas, and a gas turbine that uses the fuel gas to generate power. To produce the fuel gas required in the combined power generation unit according to a required power load, a feed-forward control of the gasification unit is performed. Also, a dead time compensator compensates for a lag and a dead time occurring while the fuel gas is fed from the gasification unit to the combined power generation unit. The gasification unit is operated using a zero flare process. The dead time compensator delays the power load required in the combined power generation unit on the basis of the lag and the dead time of the gasification unit so that the combined power generation unit is operated while a follow-up is performed with a predetermined delay.