Disclosed is an impact strength modifier (α) for a chlorine-containing resin, comprising a powder of a graft copolymer (A) obtained by graft polymerizing one or more kinds of vinyl monomers (b1) onto a polyorganosiloxane rubber (A1) or a composite rubber (A2) containing a polyorganosiloxane rubber and a polyalkyl(meth)acrylate rubber, wherein the specific surface area of the powder of the graft copolymer (A) measured by a nitrogen gas adsorption method is from 0.6 to 30 m2/g, and the pH of water used for extraction under conditions, in which (1) in an oven at 180° C., 5.0 g of the powder of the graft copolymer (A) is left to stand still and heated for 15 minutes, and (2) after the heating, the powder is dispersed in 100 ml of heated pure water and extracted for 1 hour at 70° C. while stirring, is from 4 to 11.