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 gas or oxynitriding gas, a third process gas containing a boron-containing gas, and a fourth process gas containing a carbon hydride gas. A first step performs supply of the first process gas and a preceding gas, which is one of the third and fourth process gases, while stopping supply of the second process gas and a succeeding gas, which is the other of the third and fourth process gases. A second step performs supply of the succeeding gas, while stopping supply of the second process gas and the preceding gas. A third step performs supply of the second process gas while stopping supply of the first process gas.