A controller for a gas turbine power plant, the gas turbine power plant including: 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; a fuel flow meter that measures a flow rate of fuel supplied to the combustor; an air flow meter that measures a flow rate of air drawn in the compressor; a water atomization flow meter that measures a flow rate of the atomized water supplied to the water atomization cooling apparatus; an atmospheric condition detector that measures, for example, atmospheric temperature, humidity, or pressure; and the water atomization flow rate regulating valve that controls the flow rate of the atomized water, the controller characterized by comprising: fuel flow rate control means that calculates a fuel flow rate command value to be applied to the combustor; air flow rate control means that calculates an air flow rate command value to be applied to the compressor; and water atomization flow rate control means that reads values measured by the air flow meter, the fuel flow meter, the water atomization flow meter, and the atmospheric condition detector, the fuel flow rate command value, and the air flow rate command value; calculates a water atomization flow rate correction command value that compensates for reduction in a fuel-air ratio in the combustor occurring at the combustion switching and time of day at which to start control by the water atomization flow rate correction command value; starts the water atomization cooling apparatus before the combustion switching occurs; and controls an opening amount of the water atomization flow rate regulating valve based on a water atomization flow rate command value that incorporates the water atomization flow rate correction command value starting with the control start time of day. 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; a fuel flow meter that measures a flow rate of fuel supplied to the combustor; an air flow meter that measures a flow rate of air drawn in the compressor; a water atomization flow meter that measures a flow rate of the atomized water supplied to the water atomization cooling apparatus; an atmospheric condition detector that measures, for example, atmospheric temperature, humidity, or pressure; and the water atomization flow rate regulating valve that controls the flow rate of the atomized water, the controller characterized by comprising: fuel flow rate control means that calculates a fuel flow rate command value to be applied to the combustor; air flow rate control means that calculates an air flow rate command value to be applied to the compressor; and water atomization flow rate control means that reads values measured by the air flow meter, the fuel flow meter, the water atomization flow meter, and the atmospheric condition detector, the fuel flow rate command value, and the air flow rate command value; calculates a water atomization flow rate correction command value that compensates for reduction in a fuel-air ratio in the combustor occurring at the combustion switching and time of day at which to start control by the water atomization flow rate correction command value; starts the water atomization cooling apparatus before the combustion switching occurs; and controls an opening amount of the water atomization flow rate regulating valve based on a water atomization flow rate command value that incorporates the water atomization flow rate correction command value starting with the control start time of day. fuel flow rate control means that calculates a fuel flow rate command value to be applied to the combustor; air flow rate control means that calculates an air flow rate command value to be applied to the compressor; and water atomization flow rate control means that reads values measured by the air flow meter, the fuel flow meter, the water atomization flow meter, and the atmospheric condition detector, the fuel flow rate command value, and the air flow rate command value; calculates a water atomization flow rate correction command value that compensates for reduction in a fuel-air ratio in the combustor occurring at the combustion switching and time of day at which to start control by the water atomization flow rate correction command value; starts the water atomization cooling apparatus before the combustion switching occurs; and controls an opening amount of the water atomization flow rate regulating valve based on a water atomization flow rate command value that incorporates the water atomization flow rate correction command value starting with the control start time of day.