An impact strength modifier comprises an acrylic rubber graft copolymer (I) obtained by graft-polymerizing a polyalkyl (meth)acrylate composite rubber (A) with a vinyl monomer (B), and a compound (II) which has a sulfonic acid group or a sulfuric acid group and also has two or more benzene ring skeletons, or a salt thereof. The polyalkyl (meth)acrylate composite rubber (A) contains two or more acrylic rubber components each having a different glass transition temperature, and the respective acrylic rubber components have 20% by weight or less of a monomer having two or more unsaturated bonds in a molecule. The compound (II) preferably has two or more benzene ring skeletons, and a particle size distribution of the polyalkyl (meth)acrylate composite rubber (A) is preferably a polydisperse distribution in which two or more peaks exist within a specific range. When using the impact strength modifier of the present invention, the impact resistance, particularly low-temperature impact resistance of the resin can be improved in a small amount and weatherability such as fading resistance or impact resistance of the resulting molded article can be satisfactorily maintained.