Block (co) polymer, block copolymer composition for asphalt modification, process for producing the same, and asphalt composition

A composition for asphalt modification which comprises (a) a block (co)polymer represented by general formula (I); (S-B)m-S'n (in formula (1), S and S' are each a polymer block mainly composed of an aromatic vinyl compound, B is a polymer block mainly composed of a conjugated diene, and m and n are each an integer of 0 or 1, provided that m+n is 1 or more), wherein the total combined aromatic vinyl compound content in the block (co) polymer is from more than 10% by weight to 100% by weight, and (b) a block copolymer represented by general formula (II); S-B-S, general formula (III); S-B-S' and/or general formula (IV); and (S-B)2-X (in formulas (II) to (IV), S, S' and B have the same meanings as given in general formula (I), and X is a residue of a coupling agent), at a block copolymer (a)/block copolymer (b) ratio (weight ratio) of 10-40/90-60.