The present invention is characterized in controlling the amount of cooling steam supplied to high temperature parts 8 in accordance with the dynamic characteristics of exhaust heat recovery boiler 2 when the load on gas turbine 1C is changing, in a gas turbine steam cooling system provided with an exhaust heat recovery boiler 2 for operating gas turbine 1C and generating steam using the exhaust heat from gas turbine 1C, the combined cycle plant being controlled by driving the steam turbines 3,4 by directing a steam line from exhaust heat recovery boiler 2 to steam turbines 3,4, cooling high temperature parts 8 of gas turbine 1C by branching the steam line from exhaust heat recovery boiler 2, and returning the cooled steam to the steam line.