A substrate processing apparatus capable of minimizing the effect of a filling gas in a lower space on the processing of a substrate includes: a substrate supporting unit; at least one ring surrounding the substrate supporting unit; a processing unit on the substrate supporting unit; and an exhaust unit connected to a reaction space between the substrate supporting unit and the processing unit, wherein a first gas in the reaction space is transmitted to the exhaust unit through a first channel, a second gas in a lower space below the substrate supporting unit is transmitted to the exhaust unit through a second channel, and the first channel and the second channel are separated by the at least one ring.