1. A mass flow rate-measuring method for measuring a real mass flow rate of a gas flowing through a flow passage in which a sonic nozzle having a known cross-sectional area is connected in series, said mass flow rate-measuring method comprising the steps of: previously determining relations of correspondence of a plurality of discharge coefficients classified by a physical property value of each of gases with respect to theoretical mass flow rates; measuring a pressure and a temperature of said gas on an upstream side of said sonic nozzle when said real mass flow rate of said gas flowing through said flow passage is measured; determining said theoretical mass flow rate based on said physical property value of said gas flowing through said flow passage and said measured pressure and temperature; determining said discharge coefficient by making reference to said relation of correspondence with parameters of said determined theoretical mass flow rate and said classification based on said physical property value of said gas flowing through said flow passage; and determining said real mass flow rate by obtaining a product of said determined discharge coefficient and said theoretical mass flow rate.