A semiconductor device having an insulation protection film with increased reliability and improved device characteristics, and a manufacturing method thereof which improves the planarization and reduces the interlayer capacitance of the device. The semiconductor device has a semiconductor substrate including a MOS device, a plurality of wiring regions formed on the semiconductor substrate, and a protective insulation film formed on the top layer of the wiring regions. The protective insulation film includes a first silicon oxide film, a second silicon oxide film formed on the first silicon oxide film, and a silicon nitride film composing the top layer. The process of forming the protective insulation film includes the following steps: forming the first silicon oxide film through a reaction between a silicon compound and at least one of oxygen and a compound containing oxygen by chemical vapor deposition method, forming the second silicon oxide film on the first silicon oxide film by a condensation polymerization reaction between a silicon compound and hydrogen peroxide by chemical vapor deposition, conducting an annealing treatment at a temperature of 350-500° C., and forming the silicon nitride film.