To provide a water-soluble resin additive which imparts water repellency to resins.

This copolymer is obtd. by copolymerizing 10-90wt.% fluoroalkyl (meth)acrylate and 90-10wt.% water-soluble vinyl compd. having a group selected from among carboxyl, carboxylate, sulfo, sulfonate, phosphoric and its salt groups, phenolic and its salt groups, quat. ammonium groups, amino and its salt groups, amide, formyl, and polyether-chain-contg. groups, has a number- average mol.wt. (in terms of polyethylene glycol) according to gel permeation chromatography of 2,000-200,000, and dissolves by at least 1wt.% in water with a pH of 3-12 at 30°C.