A polycarbonate-polydiorganosiloxane copolymer or a resin composition thereof having excellent impact resistance, and, in particular, excellent impact resistance at extremely low temperatures, as well as excellent chemical resistance; and a production method for the resin composition are provided. This copolymer or resin composition thereof includes a polycarbonate-polydiorganosiloxane copolymer and an optional polycarbonate resin. The copolymer includes a polycarbonate block (A-1) and a polydiorganosiloxane block (A-2). The polydiorganosiloxane block (A-2) content of the copolymer or resin composition thereof is 2.5-8.0 wt %. The copolymer or resin composition thereof satisfies (i) and (ii): (i) in a cross-sectional observation image of the copolymer or resin composition thereof that is generated using an electron beam microscope, there are 1-20 domains that have a longest diameter of at least 80 nm in an area that is 850 nm square (722,500 nm2); and (ii) average domain size is 30-100 nm.