A block copolymer that can form selectively a microphase-separated structure under exposure with an ultraviolet exposure device, and a substrate processing method by which a micropattern can be formed at a low cost on the substrate by using the block copolymer. The block copolymer has as a basic skeleton a hydrophobic block that has a repeating structure of a hydrophobic monomer and a hydrophilic block that has a repeating structure of a hydrophilic monomer having a hydrophilic functional group. At least some of the hydrophilic functional groups are covered with a hydrophobic protective group and the hydrophobic protective group, which covers the hydrophilic functional group, is dissociated from the hydrophilic functional group by light irradiation. The substrate processing method uses the block copolymer to form a micropattern on the substrate.