A thermoplastic resin composition comprising: a first copolymer including a conjugated diene-based polymer, an aromatic vinyl-based monomer unit, and a vinyl cyan-based monomer unit; and a second copolymer including an aromatic vinyl-based monomer unit and a vinyl cyan-based monomer unit, wherein the first copolymer has a degree of grafting of 32 to 50%, the second copolymer has a weight-average molecular weight of 85,000 to 115,000 g/mol, and the thermoplastic resin composition includes the first copolymer at 30 wt% or less and an oligomer residue at 1.1 wt% or less, wherein the degree of grafting of the first copolymer is as measured by the following equation after 2 g of first copolymer powder is dissolved in 300 ml of acetone while stirring for 24 hours and then centrifuged in a centrifuge, the acetone solution thus separated is added dropwise to methanol to obtain an ungrafted portion, then the ungrafted portion is dried at 60 to 120 °C to obtain dry solid, and the dry solid is weighted: Degree of grafting % = 2 g − Weight of dry solid − Weight of conjugated diene-based polymer / Weight of conjugated diene-based polymer × 100 Weight of conjugated diene-based polymer = Weight of solid of conjugated diene-based polymer theoretically added to 2 g of graft copolymer powder; or Weight of conjugated diene-based polymer as measured by analyzing a graft copolymer through infrared spectroscopy, wherein the weight-average molecular weight of the second copolymer is as measured as a relative value with respect to a standard polystyrene sample by gel permeation chromatography using tetrahydrofuran as an eluate, and wherein the oligomer residue is as determined by analysis via automatic liquid sampler-gas chromatography/flame ionization detector (ALS-GC/FID) after 2 g of a pellet-type thermoplastic resin composition is dissolved in 10 ml of chloroform and then precipitated with 30 ml of methanol to collect a supernatant, and the supernatant is filtered through a 0.2-μm disc syringe filter. a first copolymer including a conjugated diene-based polymer, an aromatic vinyl-based monomer unit, and a vinyl cyan-based monomer unit; and a second copolymer including an aromatic vinyl-based monomer unit and a vinyl cyan-based monomer unit, wherein the first copolymer has a degree of grafting of 32 to 50%, the second copolymer has a weight-average molecular weight of 85,000 to 115,000 g/mol, and the thermoplastic resin composition includes the first copolymer at 30 wt% or less and an oligomer residue at 1.1 wt% or less, wherein the degree of grafting of the first copolymer is as measured by the following equation after 2 g of first copolymer powder is dissolved in 300 ml of acetone while stirring for 24 hours and then centrifuged in a centrifuge, the acetone solution thus separated is added dropwise to methanol to obtain an ungrafted portion, then the ungrafted portion is dried at 60 to 120 °C to obtain dry solid, and the dry solid is weighted: Degree of grafting % = 2 g − Weight of dry solid − Weight of conjugated diene-based polymer / Weight of conjugated diene-based polymer × 100 Weight of conjugated diene-based polymer = Weight of solid of conjugated diene-based polymer theoretically added to 2 g of graft copolymer powder; or Weight of conjugated diene-based polymer as measured by analyzing a graft copolymer through infrared spectroscopy, wherein the weight-average molecular weight of the second copolymer is as measured as a relative value with respect to a standard polystyrene sample by gel permeation chromatography using tetrahydrofuran as an eluate, and wherein the oligomer residue is as determined by analysis via automatic liquid sampler-gas chromatography/flame ionization detector (ALS-GC/FID) after 2 g of a pellet-type thermoplastic resin composition is dissolved in 10 ml of chloroform and then precipitated with 30 ml of methanol to collect a supernatant, and the supernatant is filtered through a 0.2-μm disc syringe filter.