wherein A is an aromatic vinyl hydrocarbon polymer block having a number-average molecular weight of 10,000 through 70,000, B is a polymer block composed mainly of a conjugated diene, and X is a residual group of a coupling agent or a polyfunctional initiator such as an organo polylithium compound, and weight ratio of the aromatic vinyl hydrocarbon to the conjugated diene being 60:40 through 95:5.