When an object of treatment is subjected to, for example, a gas treatment in an airtight chamber, reaction products adhere to the inner wall surface of the chamber, an object holder therein, and the corner portions of the chamber. When a cleaning medium is injected into the chamber, according to the present invention, the reaction products are dissolved in the cleaning medium by hydrolysis. Thereafter, the cleaning medium is discharged from the chamber. Then, the chamber is heated and evacuated, so that water vapor is discharged to provide a predetermined degree of vacuum, whereupon the treatment can be started anew. Therefore, a wiping operation can be omitted. Moreover, the reaction products remaining at the corner portions of the chamber can be removed without forming a source of polluted particles, so that the necessity of overhauling can be obviated. Thus, fully automatic cleaning, so to speak, can be effected, and the chamber need not be open to the atmosphere, so that the throughput can be improved.