A method for manufacturing a semiconductor device including a lateral bipolar transistor, wherein a step of manufacturing the lateral bipolar transistor includes the steps of: forming an active region on a first conductive layer serving as a substrate, said active region having such a shape as to protrude from the first conductive layer and having an upper surface and at least one side surface in contact with the upper surface; depositing a device isolating oxide film surrounding the active region; forming a dummy gate polysilicon on the device isolating oxide film; forming an extrinsic base region in a top portion of the active region for externally connecting a base region of the active region of the lateral bipolar transistor, following a step in which an impurity is implanted in the active region for formation of a collector; after having formed the extrinsic base region, opening an open region in the device isolating oxide film, wherein the side surface of the active region is exposed by the formation of the open region; forming an intrinsic base region of the lateral bipolar transistor partially overlapping with the extrinsic base region by implanting ions in a region of the side surface of the active region exposed by the formation of the open region; depositing a polysilicon film on the semiconductor device, so as to cover the active region, the device isolating oxide film and the dummy gate polysilicon and so as to fill said open region; forming an emitter polysilicon region by etching back the deposited polysilicon film wherein, by introducing the dummy gate polysilicon, a portion of the emitter polysilicon region in contact with the side surface of the active region is shaped to be planar and to have thickness which does not decrease moving away from the side surface of the active region; forming an emitter diffusion layer on the side surface of the active region by solid-phase diffusion from the emitter polysilicon region, so that the emitter diffusion layer is at a distance from the extrinsic base layer. forming an active region on a first conductive layer serving as a substrate, said active region having such a shape as to protrude from the first conductive layer and having an upper surface and at least one side surface in contact with the upper surface; depositing a device isolating oxide film surrounding the active region; forming a dummy gate polysilicon on the device isolating oxide film; forming an extrinsic base region in a top portion of the active region for externally connecting a base region of the active region of the lateral bipolar transistor, following a step in which an impurity is implanted in the active region for formation of a collector; after having formed the extrinsic base region, opening an open region in the device isolating oxide film, wherein the side surface of the active region is exposed by the formation of the open region; forming an intrinsic base region of the lateral bipolar transistor partially overlapping with the extrinsic base region by implanting ions in a region of the side surface of the active region exposed by the formation of the open region; depositing a polysilicon film on the semiconductor device, so as to cover the active region, the device isolating oxide film and the dummy gate polysilicon and so as to fill said open region; forming an emitter polysilicon region by etching back the deposited polysilicon film wherein, by introducing the dummy gate polysilicon, a portion of the emitter polysilicon region in contact with the side surface of the active region is shaped to be planar and to have thickness which does not decrease moving away from the side surface of the active region; forming an emitter diffusion layer on the side surface of the active region by solid-phase diffusion from the emitter polysilicon region, so that the emitter diffusion layer is at a distance from the extrinsic base layer.