A controller for use in a gas turbine power plant includes a compressor that compresses combustion air; a water-atomization cooling apparatus that sprays water drops of atomized water supplied via a water-atomization flow-rate regulating valve over a flow of air drawn in the compressor; a combustor that mixes the compressed combustion air with fuel to thereby burn a fuel-air mixture and generate combustion gas at high temperature and performs combustion switching during operation; a turbine that uses the combustion gas to drive the compressor and a generator; the water-atomization flow-rate regulating valve that controls a flow rate of the atomized water; and a compressor inlet inner blade that controls a flow rate of air drawn in the compressor. The controller includes control means that calculates a fuel-air ratio correction command signal for compensating for reduction in a fuel-air ratio in the combustor occurring during the combustion switching.