Substrate processing apparatus, method of processing substrate, and method of manufacturing semiconductor device

The present invention shortens the time needed to decrease the oxygen concentration in a chamber to be filled with an inert gas to a desired concentration. A substrate processing apparatus includes: a processing chamber configured to process a substrate; and a carrying chamber configured to carry the substrate to the processing chamber. The carrying chamber includes: a plurality of wall bodies configured to form a housing of the carrying chamber; a joint at which the plurality of wall bodies are joined; an isolated space creating member configured to cover the joint and thereby create an isolated space separated from the carrying chamber; and an exhaust section configured to purge gas in the isolated space.