A device for forming a deposited film is provided. It comprises (a) a reaction chamber; (b) a heating means for heating a substrate placed in the reaction chamber; (c) a starting gas introducing means for introducing starting gases into the reaction chamber, the gas introducing means having a means for introducing two or more kinds of gases alternately and intermittently into the reaction chamber; (d) a decomposing means for decomposing the starting gases in the reaction chamber so as to form a deposited film on the substrate heated by said heating means in the reaction chamber, the decomposing means having a light source which irradiates at least one kind of light into the reaction chamber to decompose the starting gases.