Novel resin modifier and polar group-containing polymer composition including same

Disclosed is a novel resin modifier which enables to improve the low-temperature impact resistance of a polymer alloy including a polar group-containing polymer and an olefin polymer, and also enables to obtain a molded article with smooth surface when the polymer alloy is molded. Also disclosed is a polar group-containing polymer composition which contains such a resin modifier. Such a polar group-containing polymer composition may preferably be a resin composition composed of a polylactic acid and a polypropylene. Specifically disclosed is a resin modifier (C) which is obtained by reacting a polyolefin (A) having a group reactive with a carbodiimide group with a carbodiimide group-containing compound (B). The carbodiimide groups are contained in an amount of 1-200 mmol per 100 grams of the resin modifier (C). Also specifically disclosed is a polar group-containing polymer composition (F) containing 1-30 weight% of the resin modifier (C), 99-20 weight% of a polar group-containing polymer (D) and 0-80 weight%