According to the present invention, when a nitridation process by plasma generated by a microwave is applied to a substrate with an oxide film having been formed thereon to form an oxynitride film, the microwave is intermittently supplied. By the intermittent supply of the microwave, ion bombardment is reduced in accordance with a decrease in electron temperature, and a diffusion velocity of nitride species in the oxide film lowers, which as a result makes it possible to prevent nitrogen from concentrating in a substrate-side interface of an oxynitride film to increase the nitrogen concentration therein. Consequently, it is possible to improve quality of the oxynitride film, resulting in a reduced leakage current, an improved operating speed, and improved NBTI resistance.