A silicon oxide film as a dielectric film and a silicon nitride film or a polysilicon film as a protection film for the silicon oxide film are formed on a silicon substrate. After the two films are selectively etched to form contact holes of a bipolar transistor, a polysilicon film as a conductive film is laid on the entire substrate and selectively etched to form electrodes. In a MIS transistor, the protection film of the silicon nitride film serves as a gate insulator film and the protection film of the polysilicon film serves as a gate electrode. Accordingly, contamination to the gate insulator film at formation of contact holes of the bipolar transistor is prevented, and an excellent semiconductor with Bi-MOS structure is manufactured with low cost.