PURPOSE: To obtain a resin composition, good in dispersibility of an impact resistance improver and excellent in impact resistance.

CONSTITUTION: This impact improver is composed of an epoxy-modified block copolymer prepared by epoxidizing double bond in an unsaturated carbon of a conjugated diene compound of a block copolymer composed of (A) a polymer block consisting essentially of a vinyl aromatic compound and (B) a polymer block consisting essentially of the conjugated diene compound in the same molecule. Furthermore, this impact resistance improver is composed of the epoxy-modified block copolymer obtained by epoxidizing the double bond in the unsaturated carbon of the conjugated diene compound of a partially hydrogenated block copolymer prepared by partially hydrogenating the block copolymer composed of the polymer block consisting essentially of the vinyl aromatic compound and the polymer block consisting essentially of the conjugated diene compound in the same molecule. This resin composition contains the impact resistance improver.