A pulverizer that pulverizes coal into pulverized coal; a gasifier that gasifies pulverized coal pulverized by the pulverizer; a combustor that combusts a gasified gas gasified by the gasifier; a compressor that supplies compressed air to the combustor; a gas turbine driven by a combustion gas generated by the combustor; a generator driven by the gas turbine to generate power; a flue gas supply channel that guides a part of a flue gas from the gas turbine to the pulverizer; an IGV that adjusts a flow rate of air supplied from the compressor to the combustor; and a controller that applies an air flow-rate reduction operation to control the IGV so that the flow rate of air is smaller than a set air flow rate determined from a set combustion temperature of the combustor.