A film formation method for a semiconductor process is arranged to form a thin film on a target substrate by CVD, while supplying a first process gas for film formation and a second process gas for reacting with the first process gas to a process field accommodating the target substrate. The method alternately includes first to fourth steps. The first step performs supply of the first and second process gases to the process field. The second step stops supply of the first and second process gases to the process field. The third step performs supply of the second process gas to the process field while stopping supply of the first process gas to the process field. The fourth step stops supply of the first and second process gases to the process field.