It is to provide a block copolymer that can form a microphase separation structure even with a small molecular size, and that can form a microphase separation structure with a small domain size.It is a block copolymer represented by the formula A-C-B (wherein A represents a segment which is a homopolymer or random or block copolymer consisting of at least 1 kind or repeat units represented by formula (II); B represents a segment which is a homopolymer, or random or block copolymer consisting of at least 1 kind of repeat units represented by formula (III); C represents A, B or A-B; however, at least 1 segment of each A has a water-repellent group, or at least 1 segment of each B has a polar group); which block copolymer has a mass average molecular weight of 50,000 or less, and that can form a microphase separation structure.