To provide a simple method for measuring gas pressure and a low-cost device for the method applicable to a pressure reduced atmosphere and having excellent measuring accuracy and responsiveness.

This method and device for measuring the pressure of inclusion gas in mixed gas uses a quartz oscillator vacuum gauge comprising a quartz oscillator, its resonance impedance measuring means and a pressure converting means for converting the resonance impedance variation into pressure and calibrating so that the output value on base gas shows a true pressure value. The resonance impedance variation (sensitivity) Ka, Kb to the unit pressure of base gas and inclusion as are obtained, and the pressure Pa of inclusion gas is obtained by a relational expression Pa=(Pc'-Pb)/K(K=Ka/Kb) from the output value Pb of the pressure converting means when leading in only the base gas and from the output value Pc' when leading in the mixed gas. The pressure Pa is also obtained by Pa=(Pc'-Pc)/(K-1) from the output value Pc of a diaphragm type vacuum gauge on the mixed gas and the output value Pc' of the pressure converting means.