In a process chamber of a substrate processing apparatus, such as an RTP apparatus, a carrier is placed and configured to carry out a contaminant that has been attached to it. In this state, a cleaning gas containing N2 and O2 is introduced into the process chamber, and cleaning is performed under conditions including a pressure of 133.3 Pa or less and a temperature of 700° C. to 1,100° C. This cleaning is repeatedly performed by sequentially replacing a plurality of carriers.