A method for configuring the physical model of a heat recovery steam generator that can estimate the state quantity of generated steam from the state quantity of the exhaust gas to be introduced, and capable of establishing the physical model of a combined cycle power generation facility thereby. The optimum values for the flow rates Flp and Fhp, pressures and temperatures Tlp and Thp of the low pressure main steam and high pressure main steam are computed in such a way as to ensure that an objective function E stored in advance will come close to zero.