To obtain a thermoplastic resin excellent in colorbility, appearance of a molded product of it, weatherability, chemical resistance, etc., by graft- polymerizing an arom. vinyl monomer in the presence of a rubbery polymer having a specific compsn. including rigid resin particles.

A monomer mixture contg. an arom. vinyl monomer and 0.05-10wt.% vinyl monomer having a chloromethyl group is emulsion-polymerized to obtain a rigid resin particle latex of 0.01 to 0.3μm in an average particle size, in the presence of which an alkyl (meth)acrylate with a 2-12C alkyl group is polymerized to obtain (A) a graft copolymer latex. Vinyl monomers which are an alkyl (meth)acrylate with a 2-12C alkyl group and 0.05-10wt.% vinyl monomer having a chloromethyl group are emulsion-polymerized to obtain (B) a rubbery polymer latex of 0.01 to 0.3μm in average particle size. The components (A) and (B) are mixed together with A/A+B = 0.1 to 0.8 in terms of solid content to enlarge the particle size. The resulting rubbery polymer is then graft-polymerized with an arom. vinyl monomer.