PURPOSE: To eliminate dust in a chamber without disassembling it by a simple operation, and performing ashing process always in a clean state, by installing a pure water nozzle for jetting pure water against the inner surface of a chamber, and a draining means for discharging pure water scattered in a chamber.

CONSTITUTION: A feeding means 5 and an exhausting means 6 for etching gas are installed on the side parts of a chamber 1, so as to be inclined low to the chamber 1 side. A draining means 9 is equipped with a valve 9a, and keeps the chamber 1 hermetic at the time of ordinary operation. After etching gas is discharged from the chamber 1, all of the feeding and discharging system valves are closed. Next, the valve 9a of the draining means 9 is opened, and then pressurized pure water is jetted from a nozzle 8. After the jetting of the pure water is interrupted, the pure water is completely discharged for several minutes, and then the valve 9a is closed. The inside of the chamber 1 is again evacuated by the exhausting means 6. As the result of the above operation, dust in the chamber 1 can be eliminated.