A method for forming a silicon nitride film first deposits a silicon nitride film on a target substrate by CVD in a process field within a reaction container. This step is arranged to supply a first process gas containing a silane family gas and a second process gas containing a nitriding gas to the process field, and set the process field at a first temperature and a first pressure, for a first time period. The method then nitrides a surface of the silicon nitride film in the process field. This step is arranged to supply a surface-treatment gas containing a nitriding gas to the process field without supplying the first process gas, and set the process field at a second temperature and a second pressure, for a second time period shorter than the first time period.