PURPOSE: To eliminate reaction product completely which is deposited on a rear surface, etc., of a device by carrying out cleaning by moving a sample base in a space of a reduced pressure wherein a sample is plasma-processed.

CONSTITUTION: If plasma cleaning is carried out with a lower electrode 7 lifted through a driving means, reaction product on an inner surface of a discharge tube 5 and a surface of a sample holder 9 can be removed; however, that on a rear surface of a sample holder 9 and a lower electrode 7 where plasma is hard to creep can not be removed. If plasma cleaning is carried out again with the lower electrode 7 descended through the driving means, plasma is formed also between the lower electrode 7 and the sample holder 9, thereby removing reaction product on a rear surface of the sample holder 9 and the lower electrode 7 where the product can not be removed with the lower electrode 7 lifted.