To provide a graft copolymer containing a polyorganosiloxane capable of substituting a halogen or phosphorus-based flame retardant and a flame retardant resin composition excellent in flame retardant property and impact property by blending the copolymer with a thermoplastic resin.

This graft copolymer containing a polyorganosiloxane is obtained by polymerizing (D) 0-10 pts.wt, vinyl-based monomers consisting of (B) 100-50 wt. % polyfunctional monomer containing ≥2 polymerizable unsaturated bonds and (C) 0-50 wt. % other copolymerizable monomer in the presence of (A) 30-95 pts.wt. polyorganosiloxane obtained by the emulsion polymerization of a cyclic siloxane and containing (meth)acryloyloxy group, and then further polymerizing (E) 5-70 pts.wt. vinyl-based monomer. In polymerizing the (D) and/or (E), (H) a radical initiator consisting of (F) 100-30 wt. % radical initiator having >10 g/100 g solubility to water and (G) 0-70 wt. % radical initiator having 0.5-10 g/100 g solubility to water, are used. The resin composition blended with the copolymer is also provided.