A method of producing a polycarbonate-based resin composition, comprising using a polycarbonate-based resin containing a polycarbonate- polyorganosiloxane copolymer as a raw material, and the step of incorporating at least one of a styrene-based resin, a flame retardant, a flame retardant aid, and an inorganic filler into the resin, wherein the copolymer having a polycarbonate block (A-1) formed of a repeating unit represented by the following general formula and a polyorganosiloxane block (A-2) containing a repeating unit represented by the following general formula, and the copolymer using, as a raw material, a polyorganosiloxane which is obtained by blending a plurality of polyorganosiloxanes, the blend having the following features: in a differential molecular weight distribution curve obtained from measurement by gel permeation chromatography using the polystyrene calibration curve, the curve having the axis of abscissa indicating a logarithmic value log of a molecular weight M and the axis of ordinate indicating dw/dlog obtained by differentiating a concentration fraction w with respect to the logarithmic value log of the molecular weight, a dw/dlog value becomes maximum in the range of 3.4≤log ≤4.0, and a ratio of a value obtained by integrating the dw/dlog value over the range of 4.00≤log ≤4.50 to a value obtained by integrating the dw/dlog value over the entire range of the log in the differential molecular weight distribution curve is 6 to 40%, wherein the dw/dlog value is determined as defined in the description: wherein R 1 and R 2 each independently represent a halogen atom, an alkyl group having 1 to 6 carbon atoms, or an alkoxy group having 1 to 6 carbon atoms, X represents a single bond, an alkylene group having 1 to 8 carbon atoms, an alkylidene group having 2 to 8 carbon atoms, a cycloalkylene group having 5 to 15 carbon atoms, a cycloalkylidene group having 5 to 15 carbon atoms, a fluorenediyl group, an arylalkylene group having 7 to 15 carbon atoms, an arylalkylidene group having 7 to 15 carbon atoms, -S-, -SO-, -SO 2 -, -O-, or -CO-, R 3 and R 4 each independently represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or an aryl group having 6 to 12 carbon atoms, and a and b each independently represent an integer of 0 to 4, wherein the flame retardant comprises at least one selected from an organic alkali metal salt, an organic alkaline earth metal salt, a silicone-based flame retardant, and a phosphorus-based flame retardant. using a polycarbonate-based resin containing a polycarbonate- polyorganosiloxane copolymer as a raw material, and the step of incorporating at least one of a styrene-based resin, a flame retardant, a flame retardant aid, and an inorganic filler into the resin, wherein the copolymer having a polycarbonate block (A-1) formed of a repeating unit represented by the following general formula and a polyorganosiloxane block (A-2) containing a repeating unit represented by the following general formula, and the copolymer using, as a raw material, a polyorganosiloxane which is obtained by blending a plurality of polyorganosiloxanes, the blend having the following features: in a differential molecular weight distribution curve obtained from measurement by gel permeation chromatography using the polystyrene calibration curve, the curve having the axis of abscissa indicating a logarithmic value log of a molecular weight M and the axis of ordinate indicating dw/dlog obtained by differentiating a concentration fraction w with respect to the logarithmic value log of the molecular weight, a dw/dlog value becomes maximum in the range of 3.4≤log ≤4.0, and a ratio of a value obtained by integrating the dw/dlog value over the range of 4.00≤log ≤4.50 to a value obtained by integrating the dw/dlog value over the entire range of the log in the differential molecular weight distribution curve is 6 to 40%, a dw/dlog value becomes maximum in the range of 3.4≤log ≤4.0, and a ratio of a value obtained by integrating the dw/dlog value over the range of 4.00≤log ≤4.50 to a value obtained by integrating the dw/dlog value over the entire range of the log in the differential molecular weight distribution curve is 6 to 40%, wherein the dw/dlog value is determined as defined in the description: wherein R 1 and R 2 each independently represent a halogen atom, an alkyl group having 1 to 6 carbon atoms, or an alkoxy group having 1 to 6 carbon atoms, X represents a single bond, an alkylene group having 1 to 8 carbon atoms, an alkylidene group having 2 to 8 carbon atoms, a cycloalkylene group having 5 to 15 carbon atoms, a cycloalkylidene group having 5 to 15 carbon atoms, a fluorenediyl group, an arylalkylene group having 7 to 15 carbon atoms, an arylalkylidene group having 7 to 15 carbon atoms, -S-, -SO-, -SO 2 -, -O-, or -CO-, R 3 and R 4 each independently represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or an aryl group having 6 to 12 carbon atoms, and a and b each independently represent an integer of 0 to 4, wherein the flame retardant comprises at least one selected from an organic alkali metal salt, an organic alkaline earth metal salt, a silicone-based flame retardant, and a phosphorus-based flame retardant.