third dummy wafer 30 is transferred into the etching chamber 11 c in the same way as the second dummy wafer 30 and plasma cleaning is carried out after plasma cleaning is completed in the etching chamber 11 a in which the first dummy wafer 20 is placed the gate valves 15 a and 12 c are opened the used dummy wafer 30 is transferred from the etching chamber 11 a to the unload lock chamber 6 by the conveyor 14 then the gate valve 12 c is closed after the pressure of the unload lock chamber 6 is returned to the atmospheric pressure by the gas introduction device 4 the gate valve 12 d is opened the used dummy wafer 30 transferred to the unload lock chamber 6 is taken out in the air by the conveyor 13 through the gate valve 12 d and is returned to its original position in the cassette 1 c in which it is stored at the start when plasma cleaning of the etching chambers 11 b and 11 c is completed the second and third dummy wafers 20 are returned to their original positions in the cassette 1 c in this way the used dummy wafers 30 are returned to their original positions in the cassette 1 c and the dummy wafers 30 are always stocked in the cassette 1.