A block copolymer comprised of a polymer block or blocks (A) comprising conjugated diene monomer units and a polymer block or blocks (B) comprising conjugated diene monomer units and aromatic vinyl monomer units, wherein (1) the polymer block or blocks (A) have a glass transition temperature in the range of −88 to −45° C., (2) the polymer block or blocks (B) have a glass transition temperature in the range of 30 to 90° C., (3) the content of aromatic vinyl monomer units in the block copolymer is in the range of 30 to 52% by weight based on the total weight of the block copolymer, (4) the block copolymer has an aromatic vinyl block ratio of smaller than 69% by weight based on the total weight of the block copolymer, and (5) the viscosity of a 5% by weight solution of the block copolymer in styrene is at least 5 mPa·s but smaller than 30 mPa·s. This block copolymer is used as a modifier for resin.