This control device for a gas turbine connected to a power generator capable of linking to a power system is configured to be able to switch operation modes of the gas turbine between a normal operation mode in which the gas turbine rotationally drives the power generator and a reverse power operation mode in which the gas turbine is rotationally driven by the power generator actuated as a motor by receiving power from the power system, the control device comprising: an index acquisition unit configured to acquire an index indicating the pressure ratio of a compressor of the gas turbine; and a signal output unit configured to output a signal for protecting the compressor when the index becomes less than a threshold value while the gas turbine is operating in the reverse power operation.