A plant control apparatus configured to control a combined cycle power plant comprising: a first superheater configured to recover heat of exhaust gas of a first gas turbine to generate first main steam; a second superheater configured to recover heat of exhaust gas of a second gas turbine to generate second main steam; a first steam turbine configured to be supplied with the first main steam and the second main steam; a first reheater configured to heat first discharge steam obtained by dividing discharge steam of the first steam turbine to generate first reheat steam; a second reheater configured to heat second discharge steam obtained by dividing the discharge steam of the first steam turbine to generate second reheat steam; a second steam turbine configured to be supplied with the first reheat steam and the second reheat steam after the first reheat steam and the second reheat steam are merged; a first valve which is a motor-operated valve and is configured to adjust a flow rate of the first discharge steam supplied to the first reheater or the first reheat stream discharged from the first reheater; and a second valve which is a motor-operated valve and is configured to adjust a flow rate of the second discharge steam supplied to the second reheater or the second reheat stream discharged from the second reheater, wherein the plant control apparatus, comprising determination module and a controller, is configured to receive a flow rate of the first main steam measured by a first measuring instrument, a flow rate of the second main steam measured by a second measuring instrument, and a valve opening degree of the second valve measured by an opening degree meter, the determination module is configured to calculate a main steam average flow rate ratio of the measured flow rate of the first main steam and the measured flow rate of the second main steam, obtain a flow coefficient corresponding to a valve opening degree of the first valve in correspondence between a valve opening degree and a flow coefficient of the second valve, determine a target flow coefficient of the second valve by using the obtained flow coefficient and the calculated main steam average flow rate ratio, and determine, as a target opening degree of the second valve, a valve opening degree of the second valve corresponding to the target flow coefficient of the second valve in the correspondence between the valve opening degree and the flow coefficient of the second valve, and the controller is configured to compare the determined target opening degree with the measured valve opening degree of the second valve and to perform one of an opening operation and a closing operation of the second valve in accordance with a state whether the determined target opening degree is larger or smaller than the measured valve opening degree of the second valve. a first superheater configured to recover heat of exhaust gas of a first gas turbine to generate first main steam; a second superheater configured to recover heat of exhaust gas of a second gas turbine to generate second main steam; a first steam turbine configured to be supplied with the first main steam and the second main steam; a first reheater configured to heat first discharge steam obtained by dividing discharge steam of the first steam turbine to generate first reheat steam; a second reheater configured to heat second discharge steam obtained by dividing the discharge steam of the first steam turbine to generate second reheat steam; a second steam turbine configured to be supplied with the first reheat steam and the second reheat steam after the first reheat steam and the second reheat steam are merged; a first valve which is a motor-operated valve and is configured to adjust a flow rate of the first discharge steam supplied to the first reheater or the first reheat stream discharged from the first reheater; and a second valve which is a motor-operated valve and is configured to adjust a flow rate of the second discharge steam supplied to the second reheater or the second reheat stream discharged from the second reheater, wherein the plant control apparatus, comprising determination module and a controller, is configured to receive a flow rate of the first main steam measured by a first measuring instrument, a flow rate of the second main steam measured by a second measuring instrument, and a valve opening degree of the second valve measured by an opening degree meter, the determination module is configured to calculate a main steam average flow rate ratio of the measured flow rate of the first main steam and the measured flow rate of the second main steam, obtain a flow coefficient corresponding to a valve opening degree of the first valve in correspondence between a valve opening degree and a flow coefficient of the second valve, determine a target flow coefficient of the second valve by using the obtained flow coefficient and the calculated main steam average flow rate ratio, and determine, as a target opening degree of the second valve, a valve opening degree of the second valve corresponding to the target flow coefficient of the second valve in the correspondence between the valve opening degree and the flow coefficient of the second valve, and the controller is configured to compare the determined target opening degree with the measured valve opening degree of the second valve and to perform one of an opening operation and a closing operation of the second valve in accordance with a state whether the determined target opening degree is larger or smaller than the measured valve opening degree of the second valve.