The problem addressed by the present invention is to provide a block copolymer that can be used in a neutral solvent atmosphere and can produce a solid polymer membrane including nanoparticles.The problem is solved by a block copolymer represented by formula (1) below. wherein in the formula, R1 represents a C1-20 linear, branched, or cyclic alkyl group, C6-20 aryl group, or C7-20 aralkyl group; R2 represents a group having a functional group having an acid dissociation constant pKa of from 0.5 to 7; R3, R4, and R5 each show H or a C1-20 linear, branched, or cyclic alkyl group; R6, R7, and R8 each represent a hydrogen, hydroxyl group, nitro group, carboxy group, or carbonyl group; X represents an amide or ester, but may not be included; Y represents an amide or ester, but may not be included; p represents an integer of 1-10, but may not be included; n represents an integer of 3-1000; m represents an integer of 3-1000; and t represents an integer of 3-1000, but n may not be included; the arrangement of n, m, and t is arbitrary, but n and m are adjacent when n is included.