A rubber-modified styrene-based resin composition comprising (A) 70 to 92% by weight of a styrene-based polymer and 30 to 8% by weight of a rubbery polymer wherein the rubbery polymer is dispersed in the styrene-based polymer in the form of particles having an occlusion structure with a plane of symmetry, an area average particle diameter of 0.1 to 0.7 .mu.m, and a ratio of area average particle diameter to number average particle diameter of 1.0 to 2.5, the weight ratio of gel content to rubbery polymer is 1.1:1 to 4.0:1, and the swelling index is 5 to 20. The styrene-based resin composition has excellent physical properties and can be used as a material for office automation equipment, home electric appliances, sheets and so on.