A disclosed film deposition method comprises alternately repeating an adsorption step and a reaction step with an interval period therebetween. The adsorption step includes opening a first on-off valve of a source gas supplying system for a predetermined time period thereby to supply a source gas to a process chamber, closing the first valve after the predetermined time period elapses, and confining the source gas within the process tube, thereby allowing the source gas to be adsorbed on an object to be processed, while a third on-off valve of a vacuum evacuation system is closed. The reaction step includes opening a second on-off valve of a reaction gas supplying system thereby to supply a reaction gas to the process chamber, thereby allowing the source gas and the reaction gas to react with each other thereby to produce a thin film on the object to be processed.