In order to obtain a composite separation membrane which has excellent chemical resistance, separation performance and water permeability at the same time, the present invention provides a composite separation membrane comprising a separation function layer on a porous supporting membrane, characterized in that the separation function layer contains at least a polymer of a polyhedral silsesquioxane represented by the following general formula (a) and a hydrophilic monomer. (R1SiO1.5)n(R2SiO1.5)m  General Formula (a) (In the formula, R1 includes a polymerizable reactive moiety; R2 represents a hydrogen atom or an alkyl group, which may be substituted; and n and m each represent an integer satisfying n≧2 or m≧0, with n+m being 8, 10 or 12.)