PURPOSE: To increase the life of a photoelectron mask by generating gas plasma of H2Ar by high frequency wave or microwave in a subchamber, and blowing out remaining gas adhered to a wafer, thereby efficiently removing the gas in a short time.

CONSTITUTION: An RF coil 42 is disposed around a subchamber 13, the coil 42 is connected to an RF power source having 100W and 13.56MHz. After a wafer 21 is conveyed into the subchamber 13 and the subchamber 13 is closed, a turbo molecule pump 41 is operated to feed Ar gas from a gas inlet tube 43. When the subchamber 13 is evacuated 1Torr of vacuum degree, the RF power source is turned ON for 10W20sec to form a plasma in the fed Ar gas. The plasma Ar gas is energized to collide with the wafer 21 and a wafer holder 28 to blow out remaining gas adhered thereto, and the Ar remains while staying on the wafer 21 and the holder 28. The blown remaining gas is externally exhausted by the pump 41. The gas adhered to the inner walls of the subchamber 13 is blown out by the Ar.