PURPOSE: To provide a circuit detecting the contact of an air pen with a processed article and to shorten a processing treatment time.

CONSTITUTION: A magnet rod 4 is held by a holder 2 in parallel to the falling direction of the holder 2. A coil 5 covers the magnet rod 4 in a noncontact state to induct the electromotive force of electromagnetic induction due to the movement of the magnet rod 4. A detection circuit 6 is connected to the coil 5 to detect the electromotive force due to electromagnetic induction. The falling of the holder 2 is detected by detecting the electromotive force of electromagnetic induction by the detection circuit 6 and, when the electromotive force of electromagnetic induction is not detected, it is detected that the tip of an air pen 1 comes into contact with a processed article 11.