Substrate Processing Apparatus, Substrate Processing Method, Method of Manufacturing Semiconductor Device and Non-transitory Computer-readable Recording Medium

Described herein is a technique capable of suppressing an adhesion of deposits to an inside of a reaction vessel of a substrate processing apparatus. According to one aspect, there is provided a substrate processing apparatus including: a substrate retainer provided with a substrate support region; a heat insulator provided below the substrate support region; and a reaction vessel of a cylindrical shape in which the substrate retainer and the heat insulator are accommodated, wherein the reaction vessel includes: an auxiliary chamber protruding outward in a radial direction of the reaction vessel and extending along an extending direction from at least a position below an upper end of the heat insulator to a position facing the substrate support region; and a first cover provided in the auxiliary chamber along a plane perpendicular to the extending direction of the auxiliary chamber so as to divide an inner space of the auxiliary chamber.