Described herein is a technique capable of improving the uniformity of the film formation among the substrates. According to the technique described herein, there is provided a configuration including: a reaction tube having a process chamber where a plurality of substrates are processed; a buffer chamber protruding outward from the reaction tube and configured to supply a process gas to the process chamber, the buffer chamber including: a first nozzle chamber where a first nozzle is provided; and a second nozzle chamber where a second nozzle is provided; an opening portion provided at a lower end of an inner wall of the reaction tube facing the buffer chamber; and a shielding portion provided at a communicating portion of the opening portion between the second nozzle chamber and the process chamber.