Described herein is a technique capable of detecting a substrate state without contacting the substrate. According to one aspect of the technique, there is provided (a) loading a substrate retainer, where a plurality of substrates is placed, into a reaction tube; (b) processing the plurality of the substrates by supplying a gas into the reaction tube; (c) unloading the substrate retainer out of the reaction tube after the plurality of the substrates is processed; and (d) detecting the plurality of the substrates placed on the substrate retainer after the substrate retainer is rotated by a first angle with respect to a transferable position, wherein the plurality of the substrates is transferable to/from the substrate retainer in the transferable position.