A control device causes a combined cycle plant to perform a first operation section, a second operation section, and a third operation section. In the first operation section, a first gas turbine unit and a steam turbine are driven without driving an auxiliary combustion device. The control device causes the plant to perform the second operation section in which the first gas turbine unit, the auxiliary combustion device, and the steam turbine are driven when a required output equal to or greater than a maximum output in the first operation section is received. The control device causes the plant to perform the third operation section in which the first gas turbine unit, the second gas turbine unit, and the steam turbine are driven without driving an auxiliary combustion device when a required output equal to or greater than a maximum output in the second operation section is received.