Described herein is a technique capable of acquiring, monitoring and recording the progress of the reaction between a substrate and a reactive gas contained in a process gas in a process chamber during the processing of the substrate. According to the technique, there is provided a substrate processing apparatus including: a process chamber accommodating a substrate; a process gas supply system configured to supply a process gas into the process chamber via a process gas supply pipe; an exhaust pipe configured to exhaust an inner atmosphere of the process chamber; a first gas concentration sensor configured to detect a first concentration of a reactive gas contained in the process gas in the process gas supply pipe; and a second gas concentration sensor configured to detect a second concentration of the reactive gas contained in an exhaust gas in the exhaust pipe.