To obtain a resin modifier capable of imparting antistatic properties sustaining over a long period to a thermoplastic resin, especially a styrene-based resin without deteriorating thermal and mechanical characteristics thereof.

This resin modifier for antistatic use contains a reactional product of (A) a styrene-maleimide-maleic anhydride copolymer with (B) at least one polyalkylene oxide monoether selected from a polyalkylene oxide monoalkyl ether, a polyalkylene oxide monophenyl ether and a polyalkylene oxide monoalkylphenyl ether or its metal salt as an active component.