PURPOSE: To pressure-reduce a chamber even from the state of water molecules adhering to the wall surface in the device by producing plasma by feeding high-frequency power in the pressure reducing time in a plasma processor.

CONSTITUTION: A chamber 1 is exhausted at low rate by an exhaust system 7. Next, Ar gas is fed in the chamber 1 from a gas introducing system 4 while feeding microwaves from a microwave introducing port 1a. At this time, plasma can be produced in the plasma producing chamber 1b by actuating an exciting coil 2. Due to the high pressure in the chamber 1, the mean free molecular stroke of the product plasma is short to be diffused within wide range in intensive contact with the inner wall surface of the chamber 1. Due to this contact, the wall surface temperature in the chamber 1 is raised thereby enabling the adhering water molecules to be vaporized.