An insulating film is formed on a target substrate by CVD, in a process field to be selectively supplied with a first process gas containing a silane family gas, a second process gas containing a nitriding or oxynitriding gas, and a third process gas containing a carbon hydride gas. This method alternately includes first to fourth steps. The first step performs supply of the first and third process gases to the field while stopping supply of the second process gas to the process field. The second step stops supply of the first to third process gases to the field. The third step performs supply of the second process gas to the field while stopping supply of the first and third process gases to the field. The fourth step stops supply of the first to third process gases to the field.