NOVELTY - A gas-separation membrane (10) consists of a gas-separation layer (1) formed using a polyimide compound. The polyimide compound contains 2-phenyl-2,6-diazaspiroheptane-1,3,5,7-tetrone repeating unit (I).    USE - Gas-separation membrane is used in module of apparatus used for separating gas e.g. mixed gas of methane and carbon dioxide (all claimed) in thermal power station, cement plant, and iron mill blast furnace.    ADVANTAGE - The gas-separation membrane has excellent gas permeability, and enables separation of gas with high selectivity and separation performance with respect to gas or impurity of gas even at high pressure.    DETAILED DESCRIPTION - A gas-separation membrane consists of a gas-separation layer formed using a polyimide compound. The polyimide compound contains 2-phenyl-2,6-diazaspiroheptane-1,3,5,7-tetrone repeating unit of formula (I).    R=group chosen from groups of formula (ii)-(xxviii);    X1-X3=single bond or bivalent coupling group;    L=-HC=CH- or -CH2-;    R1,R2=H or substituent;    R3=alkyl, halo, carboxy, hydroxy, alkylamino, arylamino, acylamino, alkoxy, acyloxy, alkoxycarbonyl, imidazolyl, triazolyl, tetrazolyl, cyano, sulfo, sulfonamido, alkylsulfonyl, or mercapto;and    A1-A3=H, alkyl, halo, carboxy, hydroxy, alkylamino, arylamino, acylamino, alkoxy, acyloxy, alkoxycarbonyl, imidazolyl, triazolyl, tetrazolyl, cyano, sulfo, sulfonamido, alkylsulfonyl, or mercapto, in which A1-A3 are not H simultaneously.    At least 1 group of R3, A1, A2, and A3 is a group chosen from carboxy, hydroxy, alkylamino, arylamino, acylamino, alkoxy, acyloxy, alkoxycarbonyl, imidazolyl, triazolyl, tetrazolyl, cyano, sulfonamido, alkylsulfonyl, and mercapto. An INDEPENDENT CLAIM is included for separation of gas, which involves selectively-transmitting carbon dioxide from gas containing carbon dioxide and methane using the gas separation membrane.    DESCRIPTION OF DRAWING(S) - The drawing shows the cross-sectional view of gas-separation membrane.    Gas-separation layer (1)    Porous layer (2)    Gas-separation membrane (10)