PURPOSE: To prepare the titled resin having excellent thermal stability and processability, by copolymerizing styrene, acrylonitrile, methyl methacrylate, etc. in the presence of a polybutadiene latex subjected to the particle growing treatment with a specific acid group-containing copolymer latex.

CONSTITUTION: The objective resin can be obtained by polymerizing 93W 30pts.wt. of (A) a monomer mixture consisting of ≥30% styrene, acrylonitrile, and/or methyl methacrylate and ≤30% other monomer in the presence of (B) 7W 70pts.wt. of a grown rubber latex having a particle diameter of ≥0.2μ and obtained by polymerizing (i) a monomer mixture consisting of 3W30% acrylic acid, itaconic acid and/or crotonic acid, 97W35% 1W12C alkyl acrylate and 0W 48% other monomer to (ii) 100pts.wt. of a rubber latex having a pH of ≥7 and composed of 100W50% 1,3-butadiene and 0W50% monomer containing the group of formula (e.g. styrene). The polymerization of the component (i) is carried out first by polymerizing 5W90% of the monomer excepting the unsaturated acid, and then polymerizing the remaining 95W10% of the monomer.