An object of the present invention is to provide a polyorganosiloxane-containing graft copolymer capable of further improving impact resistance of a vinyl chloride-based resin without causing a significant problem such as lowering of transparency, and a vinyl chloride-based resin composition containing the copolymer. The above object is achieved by a polyorganosiloxane-containing graft copolymer (C) obtained by graft-polymerizing 65 to 5 parts by weight of at least one monomer (B) selected from the group consisting of (meth)acrylic acid ester, aromatic vinyl, and a vinyl cyanide compound under a presence of 35 to 95 parts by weight of a polyorganosiloxane (A) consisting of 0.001 to 0.5% by weight of a bifunctional siloxane-based graft-crosslinking agent (a) and 99.999 to 99.5% by weight of organosiloxane (b) (with the proviso that a total of (A) and (B) is 100 parts by weight), the graft copolymer characterized by having a degree of swelling of acetone-insoluble matter in toluene being 4000% or more; and a vinyl chloride-based resin composition containing the graft copolymer.