TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Composition: The acidic gas carrier is alkoxide of alkaline-earth metal, bicarbonate, alkaline-earth metal carbonate, alkali metal phosphate, organic amine, ionic liquid, metal complex, hydroxide of alkali metal, alkoxide of alkali metal, alkali metal carbonate, alkali metal bicarbonate, alkali metal phosphate and/or hydroxide of alkaline-earth metal. The gas mixture essentially contains carbon dioxide and methane, or carbon dioxide and hydrogen.    TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: The porous membrane is ultrafiltration membrane, reverse osmosis membrane, asymmetric membrane or nano filter. The separation active membrane is gel membrane containing polymer having crosslinked structure derived from aldehyde group, epoxide group, isocyanate group, carbodiimide group or oxazoline group. The crosslinked polymer has polyethylene glycol-type repeating unit, polypropylene glycol-type repeating unit, polyvinyl alcohol-type repeating unit, polyvinyl acetate-type repeating unit, polydimethyl siloxane-type repeating unit, polyvinylpyrrolidone-type repeating unit, polyethylene imine-type repeating unit, polyallyl amine-type repeating unit, hydroxyl group-containing repeating unit of formula: -(CH2-CH(-OH))-, carbonyl group-containing repeating unit of formula: -(CH2-C(-R1)(-J1-W1-A))-, and aromatic repeating unit of formulae: (-(CH2-C(-R4)(-J2-W2-Ra)n))-L1 and/or -(CH2-C(-R6)(-J3-W3-Rb-O-R7))-. The separation-active membrane contains polymer containing ether-type repeating units of formulae: -(CH2-CH2-O)-, -(CH2-CH2-CH2-O)-, -(CH2-HC(-CH3)-O)- and/or -(Si(-CH3)2-O)-.R1,R2,R4,R6=H or substituent;J1,J2,J3=-CO-, -COO-, -CONR2, -OCO-, -O-, methylene, phenylene or -C6H4CO-;W1,L1=bivalent coupling group;A=dissociative group;R3,R5=H or alkyl;n=2 or more;W2,W3=single bond or bivalent coupling group;Ra,Rb=alkylene glycol residue or propylene glycol residue;R8=H, alkyl, aryl or aralkyl;andR7=H, alkyl or aryl.Preferred Property: The gas-separation membrane has heat resistance of 100 degrees C or more.