To provide a method and device for adjusting a concn. of mixed gas capable of coping with the preservation of recovery rate of readily permeable gas and the suppression of power cost.

In a concn. adjusting device of the mixed gas provided with a pressurizing passage provided with pressurizing machines C11 and C12 for pressurizing the mixed gas containing ≥2 kind gases different in permeability, a passage L1 for taking out a part of the mixed gas from the pressurizing passage, a gas separation part for separating the gas supplied from the passage L1 into permeated gas and residual gas by a gas separation membrane M1 and M2 and a passage for introducing the permeated gas to the pressurizing passage, the separation part is constituted in plural stages of separation parts S1 and S2, and each stage is connected with a passage L3 for supplying the residual gas of the preceding stage to the next stage separation part S2, and passages L2 and L4 for introducing separately or after joining the permeated gas of each stage to the pressurizing passage are provided, and also the pressure difference of the pressurizing passage is used so that the pressure ratio between the supply side and the permeate side becomes larger at the more poststage side gas separation part S2.