A composition for forming a functional polymer membrane contains a monomer, which has an anionic functional group with an acid dissociation constant pKa of a, as a component A, a monomer, which has an anionic functional group with an acid dissociation constant pKa of b greater than a, as a component B, and a photopolymerization initiator represented by the following Formula PI-1 or PI-2 as a component C, in which a difference between b and a is equal to or greater than 1.5 and equal to or less than 10, and a ratio between a molar equivalent of the anionic functional group the component A has and a molar equivalent of the anionic functional group the component B has is 90:10 to 50:50.
