a film deposition method of depositing a thin film by alternately supplying at least a first source gas and a second source gas to a substrate is disclosed . the film deposition method includes steps of evacuating a process chamber where the substrate is accommodated , without supplying any gas to the process chamber ; supplying an inert gas to the process chamber until a pressure within the process chamber becomes a predetermined pressure ; supplying the first source gas to the process chamber filled with the inert gas at the predetermined pressure without evacuating the process chamber ; stopping supplying the first source gas to the process chamber and evacuating the process chamber ; supplying the second source gas to the process chamber ; and stopping supplying the second source gas to the process chamber and evacuating the process chamber .