To provide a polymeric emulsifying and dispersing agent used in a rosinous emulsion sizing agent and having excellent emulsifying stability and sizability.

A copolymer mixture formed by mixing the styrene-(meth)acrylic acid copolymers having different weight average molecular weights on the level exhibiting a plurality of peaks on a molecular weight distribution curve in a gel permeation chromatogram is saponified to produce an aqueous alkali salt solution, and the aqueous copolymer-salt solution is used as a polymeric emulsifying and dispersing agent. Since the copolymers with the molecular diffusion coefficient varied coexist in the copolymer mixture exhibiting a plurality of peaks, the agent is excellent in rosin emulsion emulsifying property as compared with the one based on a copolymer exhibiting only a single peak and further paper excellent in sizing properties is manufactured by using the rosin emulsion as an internal sizing agent.