A method of manufacturing a semiconductor device by processing a substrate by supplying a processing space with a gas dispersed in a buffer space disposed at an upstream side of the processing space is provided. The method includes (a) transferring the substrate into the processing space while exhausting a transfer space of the substrate by a first vacuum pump; (b) closing a first valve disposed at a downstream side of the first vacuum pump; (c) supplying the gas into the processing space via the buffer space; and (d) exhausting the buffer space through an exhaust pipe connected to a downstream side of the first valve.