Mixed gas concentration regulating method and concentration regulating apparatus

A mixed gas concentration regulating method and a concentration regulating apparatus, in which the maintenance of the recovery of an easily permeable gas and the restraint of the power expense are compatible. The mixed gas concentration regulating apparatus which includes a pressure raising line having boosters for raising the pressure of a mixed gas having two or more gases different in permeability, a line for leading out a part of the mixed gas from the pressure raising line, a gas separation section for separating the gas fed from the line to a permeate gas and a retentate gas by gas separation membranes, and a line for introducing the permeate gas into the pressure raising line, wherein the gas separation section includes plural stages the respective stages being connected with each other by a line for feeding a retentate gas of the preceding stage to a gas separation section of the succeeding stage and lines are provided for introducing permeate gases of the respective stages into the pressure raising line separately or after they are joined, and the pressure difference of the pressure raising line is utilized so that the pressure ratio on the feed side and permeation side is made larger as in the gas separation section of the latter stage side.