Abstract: A cleaning gas is supplied into a processing chamber defined by a processing vessel included in a film forming system, and the temperature of a gas contained in the processing vessel is measured continuously for a predetermined time by a gas temperature measuring means disposed in the processing vessel. The temperature measured by the gas temperature measuring means rises gradually while the cleaning gas flowing through the processing chamber is reacting with an unnecessary film deposited on the surfaces of the processing vessel. A cleaning operation terminating point is determined on the basis of the time-varying measured temperature of the gas measured by the gas temperature measuring means. Preferably, the cleaning operation terminating point determining means determines that the cleaning operation is to be terminated at a time point a certain time after the variation of the measured temperature past a peak.