A control device includes a mode-recognizing unit that recognizes whether a start mode of a steam turbine is a cold mode in which a temperature of a steam contact portion of the steam turbine is lower than a predetermined temperature or a different start mode in which the temperature of the steam contact portion is equal to or higher than the predetermined temperature, a first opening generator that generates a first degree of opening greater than a second degree of opening in the different start mode as a degree of opening of an intake air regulator of a compressor in a period after generation of steam from an exhaust heat recovery boiler is started and before supplying of the steam to the steam turbine is started, and a command output unit that outputs a command corresponding to the first degree of opening to the intake air regulator.