An object of the present invention is to provide a modifier for styrene resins that has excellent powder characteristics and that can provide a styrene resin composition for a styrene resin having high impact resistance and formability when the modifier is contained in a small amount.A modifier (C) for styrene resins that has improved powder characteristics can be obtained by mixing, in advance or in advance in latex form, 1 to 400 parts by weight of a styrene copolymer (B) with 100 parts by weight of a polyorganosiloxane-containing graft copolymer (A) having a polyorganosiloxane backbone produced by polymerizing an organosiloxane and a grafting agent, and then granulating the resulting mixture. Thus, the above-described object can be achieved.