A substrate processing apparatus according to the present invention promotes supplying gases to spaces between adjacent substrates without reducing the number of substrates which can be collectively processed. The substrate processing apparatus includes: a processing chamber for storing and processing substrates stacked in multiple stages in horizontal posture; at least one processing gas supply nozzle which extends running along an inner wall of the processing chamber in the stacking direction of the substrates and supplies a processing gas to the inside of the processing chamber; a pair of inactive gas supply nozzles which are provided so as to extend running along the inner wall of the processing chamber in the stacking direction of the substrates and so as to sandwich the processing gas supply nozzle from both sides thereof along the circumferential direction of the substrates and which supply the inactive gas to the inside of the processing chamber; and an exhaust line for exhausting the inside of the processing chamber.