According to one aspect of the present disclosure, there is provided a technique that includes: a substrate retainer; a reaction tube; a heater configured to heat an inside of the reaction tube; a gas supplier configured to supply a process gas to substrates accommodated in the reaction tube; an exhauster configured to exhaust the process gas from the inside of the reaction tube; a temperature detector configured to measure an inner temperature of the reaction tube; a reflectance detector configured to measure a reflectance of a film formed by supplying the process gas through the gas supplier; and a controller configured to be capable of performing a feedback control of film-forming conditions on the substrates accommodated in the reaction tube by using temperature information measured by the temperature detector and reflectance information measured by the reflectance detector.