A substrate processing method includes: disposing a wafer in a wafer region of a tube; injecting an inert gas into a gap region, of the tube, between an inner side wall of the tube and the wafer disposed in the wafer region; and injecting a process gas into the wafer region of the tube, wherein a pressure of the gap region of the tube is higher than a pressure at an edge of the wafer region of the tube during the injection of the inert gas and the process gas.