NOVELTY - A gas-separation composite film (10) has a gas-separation layer (1) containing polyimide compound, on an upper side of a gas-permeable support layer. The polyimide compound consists of repeating unit (a) (I)-(III) and a repeating unit (IV). The content of repeating unit (a) contained with respect to total content of repeating units (a) and (b) is 10-90 mol%.    USE - Gas-separation composite film is used for gas-separation module used in apparatus for separation of gas e.g. carbon dioxide from mixture of carbon dioxide and methane (all claimed).    ADVANTAGE - The gas separation composite film has excellent gas permeability and high gas-separation performance and gas-separation selectivity. The gas containing impurity components (e.g. toluene) can be separated even in high pressure conditions.    DETAILED DESCRIPTION - A gas-separation composite film has a gas-separation layer containing polyimide compound, on an upper side of a gas-permeable support layer. The polyimide compound consists of repeating unit (a) of formulae (I)-(III) and a repeating unit of formula (IV). The content of repeating unit (a) contained with respect to total content of repeating units (a) and (b) is 10-90 mol%.    R=group chosen from groups of formulae (1)-(28);    X1-X3,Yc=single bond or bivalent coupling group;    L=-CH=CH- or -CH2-;    one of three Xa=-N= and other being -CR'a=;    R'a,,R'b,R1,R2=H or substituent;    R'c=substituent;    nc1=0-3;and    Ld=bivalent aliphatic group, bivalent aryl or bivalent group with which two or more aryl are connected by single bond or bivalent coupling group.    INDEPENDENT CLAIMS are included for the following:    (1) gas-separation module, which comprises the gas-separation composite film;    (2) gas-separation apparatus, which is provided with the gas-separation module; and    (3) gas-separation method, which involves transmitting carbon dioxide selectively from gas containing carbon dioxide and methane using the gas-separation composite film.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the gas-separation composite film.    Gas-separation layer (1)    Porous layer (2)    Gas-separation composite film (10)