A substrate processing method for use in a substrate processing apparatus having a stocker therein which stores a multiplicity of dummy substrates; a reaction chamber for producing semiconductor products; and a transferring unit for transferring into the reaction chamber a process substrate and the dummy substrate stored in the stocker in order to form a film on the process substrate, the method includes transferring one dummy substrate selected among the dummy substrates stored in the stocker to the reaction chamber without being out of the apparatus; and introducing a cleaning gas into the reaction chamber to clean said one dummy substrate within the reaction chamber.