Described herein is a technique capable of improving a film uniformity on a surface of a substrate and a film uniformity among a plurality of substrates including the substrate. According to one aspect thereof, there is provided a substrate processing apparatus including: a substrate retainer including: a product wafer support region, an upper dummy wafer support region and a lower dummy wafer support region; a process chamber in which the substrate retainer is accommodated; a first, a second and a third gas supplier; and an exhaust system. Each of the first gas and the third gas supplier includes a vertically extending nozzle with holes, wherein an upper of an uppermost hole and a lower end of a lowermost hole are arranged corresponding to an uppermost and a lowermost dummy wafer, respectively. The second gas supplier includes a nozzle with holes or a slit.