A gas turbine plant including a gas turbine and a compressor is provided with a steam turbine plant including a steam turbine and a condenser, and, an exhaust heat recovery boiler. Steam from the exhaust heat recovery boiler is directly flown to the condenser of the steam turbine plant through a bypass control valve. A pressure sensor detects pressure in a turbine bypass system. A controller outputs, based on a set value from an input device and a process value from the pressure sensor, an open level instruction value to the control value so as to make the process value consistent with the set value in a predetermined sampling cycle. A corrector corrects the set value from the input device in a direction in which the open level instruction value decreases when the open level instruction value from the controller becomes a value that substantially fully opens the control valve.