The rubber-modified styrene-based copolymeric resin of the invention comprises a matrix phase of a copolymer of styrene and acrylonitrile and a dispersed phase of particles of an S-B type block-copolymeric rubber of styrene and butadiene as prepared by the bulk-suspension polymerization method or continuous bulk polymerization of the monomers in the presence of the rubber. The rubber-modified styrene-based copolymeric resin is imparted with excellent rigidity, impact strength and appearance, in particular, in welded portions when it is prepared by adequately selecting several parameters including the weight proportion of the moieties derived from styrene and acrylonitrile in the matrix phase and content of styrene, solution viscosity, content of 1,2-unsaturated linkages, particle size and distribution thereof in the rubber constituting the dispersed phase.