A process for producing a rubber-modified thermoplastic resin, which comprises polymerizing (B) 30-100% by weight of an aromatic alkenyl compound monomer and/or an alkyl (meth)acrylate monomer and (C) 70-0% by weight of at least one other monomer compound copolymerizable therewith, in the presence of (A) a hydrogenated diene copolymer obtained by hydrogenating a diene copolymer comprising a rndom copolymer of 50% by weight or more of at least one conjugated diene and 50% by weight or less of an alkenyl aromatic compound until at least 70% by weight of the olefinic unsaturation of the diene copolymer is hydrogenated. The hydrogenated diene copolymer (A) may be a hydrogenated copolymer consisting of an A-B block copolymer or an A-B-C block copolymer (A is an alkenyl aromatic compound polymer block, B is an alkenyl aromatic compound-conjugated diene random copolymer block, and C is a polymer block composed mainly of an alkenyl aromatic compound), wherein(i) the weight ratio of the alkenyl aromatic compound/the conjugated diene is 5-50/95-50,(ii) the total amount of the alkenyl aromatic compounds of the block A and the block C in the copolymer is 3-40% by weight, and(iii) the vinyl configuration content in the conjugated diene portion of the block B is 10-90%, and wherein at least 90% of the double bonds of the conjugated diene is saturated.