A composition for forming a functional polymer membrane, comprising: 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, wherein the acid dissociation constants of the anionic functional groups of the components A and B are calculated in accordance with the conditions and using the software as described in the description, a difference between b and a is equal to or greater than 1.5 and equal to or less than 10, 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, in Formula PI-1, R p1 represents an alkyl group, an alkenyl group, an alkoxy group, or an aryloxy group, R p2 and R p3 each independently represent a hydrogen atom, an alkyl group, an alkenyl group, an alkoxy group, or an aryloxy group, R p2 and R p3 may form a ring by being bonded to each other, and n represents an integer of 0 to 5, in Formula PI-2, R p4 represents an alkyl group, an aryl group, an alkylthio group, or an arylthio group, R p5 represents an alkyl group, an aryl group, an alkylthio group, an arylthio group, or an acyl group, and R p6 represents an alkyl group or an aryl group, wherein the component A is a monomer having at least one anionic functional group selected from the group consisting of a sulfonic acid group, a phosphonic acid group, and a phosphoric acid group, and the component B is a monomer having a carboxylic acid group. 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, wherein the acid dissociation constants of the anionic functional groups of the components A and B are calculated in accordance with the conditions and using the software as described in the description, a difference between b and a is equal to or greater than 1.5 and equal to or less than 10, 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, in Formula PI-1, R p1 represents an alkyl group, an alkenyl group, an alkoxy group, or an aryloxy group, R p2 and R p3 each independently represent a hydrogen atom, an alkyl group, an alkenyl group, an alkoxy group, or an aryloxy group, R p2 and R p3 may form a ring by being bonded to each other, and n represents an integer of 0 to 5, in Formula PI-2, R p4 represents an alkyl group, an aryl group, an alkylthio group, or an arylthio group, R p5 represents an alkyl group, an aryl group, an alkylthio group, an arylthio group, or an acyl group, and R p6 represents an alkyl group or an aryl group, wherein the component A is a monomer having at least one anionic functional group selected from the group consisting of a sulfonic acid group, a phosphonic acid group, and a phosphoric acid group, and the component B is a monomer having a carboxylic acid group. R p2 and R p3 each independently represent a hydrogen atom, an alkyl group, an alkenyl group, an alkoxy group, or an aryloxy group, R p2 and R p3 may form a ring by being bonded to each other, and n represents an integer of 0 to 5, R p5 represents an alkyl group, an aryl group, an alkylthio group, an arylthio group, or an acyl group, and R p6 represents an alkyl group or an aryl group, wherein the component A is a monomer having at least one anionic functional group selected from the group consisting of a sulfonic acid group, a phosphonic acid group, and a phosphoric acid group, and the component B is a monomer having a carboxylic acid group.