In one embodiment, a combined cycle power plant includes first and second superheaters to generate first and second main steams, first and second reheaters to heat first and second discharge steams to generate first and second reheat steams, and a steam turbine to be supplied with the merged first and second reheat steams. The plant further includes a first valve to adjust a flow rate of the first discharge or reheat steam, and a second valve to adjust a flow rate of the second discharge or reheat steam. A plant control apparatus includes a determination module to determine a target opening degree of the second valve by using flow rates of the first and second main steams, and a controller to compare the determined target opening degree with a valve opening degree of the second valve and to control the second valve based on a comparison result.