A silicon oxynitride film is formed on a target substrate by CVD, in a process field configured to be selectively supplied with a first process gas containing a chlorosilane family gas, a second process gas containing an oxidizing gas, and a third process gas containing a nitriding gas. This method alternately includes first to sixth steps. The first, third, and fifth steps perform supply of the first, second, and third process gases, respectively, while stopping supply of the other two process gases. Each of the second, fourth, and sixth steps stops supply of the first to third process gases. The third and fifth steps include an excitation period of supplying the second and third process gases, respectively, to the process field while exciting the respective process gases by an exciting mechanism.