When a gas standard mixture is consumed, the pressure in its cylinder is reduced. We studied how much this affects the concentrations of the gas standard mixtures. We prepared VOCs (Volatile Organic Compaunds) gas standard mixtures (100 vol ppb), and analyzed their concentrations at 10, 8, 6, 4, 2, 1 MPa each by GC-FID. As a result, the concentrations were not changed in 3 components (dichloromethane, 1,3-butadiene, vinyl chloride). However, the concentrations increased with the consumption in 5 components (benzene, chloroform, 1,2-dichloroethane, trichloroethylene, tetrachloroethylene). 1,2-dichloroethane had the largest increase, and its concentrations in 1 MPa was 3% higher than that in 10 MPa. Therefore the pressure should be checked when using a gas standard mixture. Also, it should be renewed if necessary.