A plurality of wafers each having a conductive film on each face thereof are housed in slots of a cassette, and a potential of "H" level is applied from a control box to each wafer via an electrode. A conductive suction part of a vacuum pincette is connected to a grounding potential. A LED corresponding to the wafer to be operated on is lit up by a specification from a computer so that an operator operates on the wafer specified by the computer using the vacuum pincette. The control box detects the potential of each electrode and decides as to whether a given wafer is correctly operated on or not. In the case of erroneous operation, such an erroneous operation is displayed by means of a buzzer, and an overall result of operation is stored in the computer.