A substrate processing apparatus includes an inner tube installed to accommodate a plurality of substrates and having a first opening, an outer tube configured to surround the inner tube, a movable wall installed inside the inner tube or between the inner tube and the outer tube, configured to be movable, and having a second opening, a gas supply part configured to supply a process gas to the substrates and an exhaust part installed outside the movable wall and configured to exhaust the process gas supplied to the substrates.