A method for manufacturing a semiconductor device in which a lateral bipolar transistor and a CMOS transistor are hybrid-integrated on a single chip, the method comprising the steps of: forming an extrinsic base region for externally connecting an intrinsic base region of an active region of the lateral bipolar transistor, following a step of forming a doped semiconductor region of the CMOS transistor in which an impurity is implanted with the gate electrode of the CMOS transistor used as a mask; forming an open region adjacent the active region by removing a predetermined portion of a device isolating oxide film surrounding the active region thereby exposing a side surface of the active region; forming the intrinsic base region of the lateral bipolar transistor by implanting ions in a region of the side surface of the active region exposed by the formation of the open region, the region of the side surface in which ions are implanted partially overlapping with the extrinsic base region; forming a protective film on the exposed surface of the active region for protecting the active region during polysilicon etching in a subsequent step; forming an impurity-containing polysilicon film in the open region and on a region including the active region on 3 which the protective film is formed in the protective film formation step; etching away the polysilicon film by a predetermined thickness; forming an emitter diffusion layer region by solid-phase diffusing an impurity into the side surface of the active region from the impurity-containing polysilicon film left after etching in the polysilicon etching step, through the protective film; forming a collector contact region in the active region by ion implantation; and forming an electrode (342, 344) directly connected to the collector contact region. forming an extrinsic base region for externally connecting an intrinsic base region of an active region of the lateral bipolar transistor, following a step of forming a doped semiconductor region of the CMOS transistor in which an impurity is implanted with the gate electrode of the CMOS transistor used as a mask; forming an open region adjacent the active region by removing a predetermined portion of a device isolating oxide film surrounding the active region thereby exposing a side surface of the active region; forming the intrinsic base region of the lateral bipolar transistor by implanting ions in a region of the side surface of the active region exposed by the formation of the open region, the region of the side surface in which ions are implanted partially overlapping with the extrinsic base region; forming a protective film on the exposed surface of the active region for protecting the active region during polysilicon etching in a subsequent step; forming an impurity-containing polysilicon film in the open region and on a region including the active region on 3 which the protective film is formed in the protective film formation step; etching away the polysilicon film by a predetermined thickness; forming an emitter diffusion layer region by solid-phase diffusing an impurity into the side surface of the active region from the impurity-containing polysilicon film left after etching in the polysilicon etching step, through the protective film; forming a collector contact region in the active region by ion implantation; and forming an electrode (342, 344) directly connected to the collector contact region.