PURPOSE: To judge exactly the end of cleaning in the stage of generating plasma in a vacuum vessel of a sputter etching device or the like and cleaning the vessel, by flowing gaseous O2 into the vessel.

CONSTITUTION: When the vacuum vessel 1 of, for example, plasma CVD device repeats CVD treatment, a hydrocarbon polymer, etc. stick gradually on the inside wall thereof and contamination progresses. The contaminants can be removed by the cleaning wherein the contaminants are liberated from the wall surfaces by the plasma generated between electrodes 2, 2 in the absence of the work 6 and are discharged through an evacuation port 3, but the end point of the cleaning is uncertain. Thereupon, gaseous O2 is introduced through a circuit 7 into the vessel 1 to create an O2 atmosphere and plasma is generated, then the intensity of the emission wavelength of the O atom radicals in the vessel 1 is detected with a monochrometer 9 or the like through a quartz window 8 provided on one side of the vessel 1. The intensity is low in the initial period of the cleaning but it rises once around the end and remains constant after the end and therefore the completion of the cleaning is judged.