A modifier for resin and rubber of which an active ingredient is a reaction product of a styrene-maleic anhydride copolymer (A) with a poly(alkylene oxide) monoalkyl ether (B), and/or a metal salt thereof, which modifier improves processability and properties of resins or rubbers. For example, the modifier provides resins or rubbers with antistatic properties. It also enhances the dispersibility of additives, e.g., a flame retardant or fillers incorporated in the resin or the rubber, and provides resins or rubbers with, for example, excellent flame retardancy and prevents the additive or the filler from blooming.