The present invention relates to a novel polyorganosiloxane capable of producing a copolycarbonate having improved hardness and to a copolycarbonate prepared by using the same. The novel polyorganosiloxane according to the present invention can be used as a monomer of a copolycarbonate, and it can exhibit improved hardness and chemical resistance simultaneously while maintaining the intrinsic properties of copolycarbonate due to the alkylene or isosorbide-derived structure included in the formula thereof.