A processing aid for a thermoplastic resin is disclosed, which is a two-stage polymer obtained by polymerizing (B) 5 to 50 parts by weight of a monomer or monomer mixture composed mainly of methyl methacrylate in the presence of (A) 50 to 95 parts by weight of a polymer comprising 100 to 50% by weight of units of a methacrylic acid ester other than methyl methacrylate and 0 to 50% by weight of units of other monomer copolymerizable with the methacrylic acid ester and having a reduced viscosity .eta.sp/C not larger than 2.0 dl/g, so that the total amount of the components (A) and (B) is 100 parts by weight and the reduced viscosity .eta.sp of the final polymer is not larger than 2.0 dl/g. A thermoplastic resin having the processing aid incorporated therein has an improved shaping processability, and a shaped article mode therefrom has good surface characteristics.