NOVELTY - A gas separation membrane has separation active membrane containing nitrogen-containing compound (I) with boiling point or decomposition temperature of 200 degrees C or more, and crosslinked polymer containing repeating unit derived from dissociative group and alkylene glycol. The ratio of sum of total molecular weight of the primary amino group and total molecular weight of the secondary amino group with respect to the molecular weight of compound (I) is 0.3-0.84.    USE - Membrane used in membrane module for apparatus used for separating acidic gas mixture from mixture of gases (all claimed).    ADVANTAGE - The gas separation membrane efficiently separates acidic gas from gaseous mixture containing perfluoro compounds e.g. tetrafluoroethane, and unsaturated hydrocarbon e.g. propylene. The membrane has excellent heat resistance, stability, mechanical strength and reduced carrier elution even under high temperature and high humidity conditions.    DETAILED DESCRIPTION - A gas separation membrane has separation active membrane containing nitrogen-containing compound of formula: R1-NH-W1-N(-R2)-R3 (I) with boiling point or decomposition temperature of 200 degrees C or more, and crosslinked polymer containing repeating unit derived from dissociative group and alkylene glycol. The ratio of sum of total molecular weight of the primary amino group and total molecular weight of the secondary amino group with respect to the molecular weight of compound (I) is 0.3-0.84.    R1-R3=H or substituent;and    W1=bivalent coupling group.    (i) When R1-R3 are substituent, R1+R2, R1+R3, or R2+R3 form a ring. INDEPENDENT CLAIMS are included for the following:    (1) manufacture of the gas separation membrane, which involves thermosetting membrane containing polymer having repeating unit of formula: -(CH2-C(-R4)(-J1-W2-A))a- (i), -(CH2-C(-R5)(-J2-W3-Y1-L1))b- (ii) or -(-(CH2-C(-R6)(-J3-W4-Y2-R7))c-)x (iii) and the compound (I);    (2) gas separation method, which involves separating acidic gas e.g. carbon dioxide, nitrogen sulfide, carbon disulfide, sulfur oxide and nitrogen oxide from mixture of acidic gas and non-acidic gas e.g. hydrogen, methane, nitrogen and carbon monoxide;    (3) gas separation membrane module, which contains the gas separation membrane; and    (4) gas permeation apparatus containing the module.    R4-R7=H or methyl;    J1-J3=-CO-, -COO-, -CONR8-, -OCO-, -O-, methylene, phenylene or -C6H4CO-;    R8=R4;    W2-W4=single bond or bivalent coupling group;    A=dissociative group;    Y1,Y2=alkylene glycol residue;    a-c=1 or more;    x=0 or more;and    L1=n valent coupling group.