To provide a new block copolymer capable of remarkably improving adhesion between a molding consisting of a non-polar polymer, such as polypropylene and polyethylene, and a coating material or a molding consisting of a polymer having a polarity, and to provide its manufacturing process.

The block copolymer consists of at least one aromatic vinyl polymer block having a weight average mol. wt. of 1,000-500,000 and at least one block of a cyclization product of a conjugated diene polymer. A modifier for a polymer molding material, which contains the above block copolymer as the effective component, is provided.