NOVELTY - A gel-like thin film (1) comprises a polymer obtained by performing ene-thiol reaction of thiol group-containing compound (I) and divalent organic group-containing compound (II), and liquid including ionic liquid, glycerol, polyglycerol, polyethylene glycol and/or polypropylene glycol.    USE - Gel-like thin film is used for manufacturing composite film for gas permeation membrane (all claimed) for separating and recovering carbon dioxide and nitrogen, and carbon dioxide and methane from off-gas of oil field, waste gas of refuse incineration and natural gas.    ADVANTAGE - The gel-like thin film has high liquid content rate, strength and pressure resistance, is manufactured in a short period of time, and provides gas permeation membrane having high gas permeability and separation property of carbon dioxide in high and low pressure conditions.    DETAILED DESCRIPTION - A gel-like thin film comprises a polymer obtained by performing ene-thiol reaction of thiol group-containing compound of formula: X1-(O-(CH2CH2O)n1-Y1-SH)m1 (I) and divalent organic group-containing compound of formula: X2-(O-(CH2CH2O)n2-Y2-R2)m2 (II), and liquid including ionic liquid, glycerol, polyglycerol, polyethylene glycol and/or polypropylene glycol.    n1,n2=15-250;    m1,m2=3-20;    X1=monovalent organic group;    Y1=1-15C hydrocarbyl, -CO-R11-, -CO-NH-R11-, -R12-O-R13-, -R12-NH-R13-, -R12-CO-R13-, -R12-COO-R13-, -R12-CONH-R13- or -R12-O-CO-NH-R13-;    R11-R13=1-10C hydrocarbyl;    X2=divalent organic group;    Y2=1-15C hydrocarbyl, -CO-, -CO-NH-, -CO-R21-, -CO-NH-R21-, -R22-O-R23-, -R22-NH-R23-, -R22-CO-R23-, -R22-COO-R23-, -R22-CO-NH-R23- or -R22-O-CO-NH-R23-;    R21-R23=R11;and    R2=monovalent organic group having carbon-carbon double bond.    INDEPENDENT CLAIMS are included for the following:    (1) composite film, which includes a porous support and the gel-like thin film laminated on the surface of the porous support;    (2) manufacture of gel-like thin film, which involves performing ene-thiol reaction with light and/or heat; and    (3) manufacture of composite film, which involves coating the solution containing the compound (I) and the compound (II) on the surface of porous support, and performing ene-thiol reaction on the coated surface.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic cross-sectional view of the gel-like thin film.    Gel-like thin film (1)