A pressure-type flow rate control device 1, while maintaining an upstream pressure P1 of an orifice 5 at approximately at least twice a downstream pressure P2, calculates a flow factor FF of a mixed gas consisting of two types of gases mixed at a mixture ratio of X:(1−X) by FF=(k/ρ){2/(κ+1)}1/(κ−1)[κ/{(κ+1)R}]1/2 using an average density ρ, an average specific heat ratio κ, and an average gas constant R of the mixed gas that are calculated by weighting the densities, specific heat ratios, and gas constants of the two types of gases at the mixture ratio, and calculates a flow rate Q of the mixed gas passing through the orifice by Q=FF·S·P1(1/T1)1/2, where S is the orifice cross section, and P1 and T1 are respectively the pressure and temperature of the mixed gas on the upstream side of the orifice.