A method of cleaning an interior of a process chamber by supplying a cleaning gas into the process chamber after a process of forming a thin film on a substrate in the process chamber is performed, including alternately repeating changing a pressure in the process chamber from a first pressure range to a second pressure range, and changing the pressure in the process chamber from the second pressure range to the first pressure range. In this method, when the pressure in the process chamber is changed to the first pressure range, the pressure in the process chamber is changed to the first pressure range without being maintained at the second pressure range, and when the pressure in the process chamber is changed to the second pressure range, the pressure in the process chamber is changed to the second pressure range without being maintained at the first pressure range.