A method of cleaning a powdery source supply system prevents outflow of particles from a chamber or an introduction line in a film forming process. A substrate processing system includes a powdery source supply system and a film forming processing unit. The powdery source supply system includes an ampoule for accommodating a powdery source, a carrier gas supply unit for supplying a carrier gas into the ampoule, an introduction line for connecting the ampoule and the film forming processing unit, a purge line branched from the introduction line, and a valve for opening or closing the introduction line. When the valve is opened and the interior of the purge line is evacuated prior to the film forming process, the carrier gas supply unit supplies a carrier gas so that the viscous force acting on particles by the carrier gas is greater than the viscous force in the film forming process.