While the polymer block A of the block copolymer (a) is preferably composed solely of structural units derived from ??-methylstyrene, the polymer block A may contain a small amount, preferably 10% by mass or less relative to the polymer block A, of one or two or more of structural units derived from unsaturated monomers other than ??-methylstyrene, such as butadiene, isoprene, 2,3-dimethyl-1,3-but