To obtain both a polymer material useful as a modifier for polymers having general-purpose properties capable of improving physical properties of polymers to be modified without causing reduction in fluidity, gelatinization, etc., of polymers and a modifier for polymers comprising this polymer material.

This block copolymer comprises a polyamide (I) block and a block (II) derived from (1) and/or (2) block copolymer. (1) The block polymer is composed of a polymer block (A) constituted of one or more of an aromatic vinyl compound polymer block and a polybutadiene block having <30 % hydrogenated 1,2-bond amount and a polymer block (B) constituted of one or more of a hydrogenated polyisoprene block, a polybutadiene block having 30-80% hydrogenated 1,2-bond amount and a hydrogenated isoprene/butadiene copolymer block. (2) The block copolymer is composed of an aromatic vinyl compound polymer block and a polyisobutylene block.