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.