PURPOSE: To improve the controllability for gas pressure by making an output to follow a target value in response to a change of the target value of a generated output for holding the pressure constant while precedingly changing gas generation quantity in accordance with variation of the generated output in consideration of the quantity of gas to be necessary at the time when pressure difference is generated.

CONSTITUTION: In the power generating plant in the title, having a gas turbine 9 using coal gas generated in a gasifying furnace 3 as a fuel and a steam turbine 15 using heat due to exhaust gas of a gas turbine, a generated output control unit (gas turbine control unit) 21 for controlling a fuel flow adjusting value 6 is provided in order to make the generated output to follow target value thereof by receiving the target value of the generated output. Besides, a pressure control unit (gasifying furnace control unit) 20 for controlling a slurry supply pump 2 and an oxygen flow adjusting valve 1 and holding gas pressure to a set value is provided. Furthermore, a gas flow setting unit (load control unit) 18 for predicting gas system pressure distribution change occurring due to change in gas flow at a time of changing the target value of the power generation output and calculating gas flow rate corresponding to the pressure distribution at the time is provided.