To provide an air pen which detects the vibration condition of a stylus from the change in the light receiving output of a photoelectric switch.

When compressible fluid is supplied to the holder 1 of an air pen, a piston 2 vibrates at a high speed, a stylus 2A at the tip of the piston 2 pushes a work, and the work is engraved by moving the holder 1. A photoelectric switch 4 which detects the presence/absence of the piston 2 by the light reflection to the periphery of the piston 2 is embedded in the side wall of the holder 1. The frequency of the variation output voltage of the switch 4 is converted into voltage by an F/V converter 5, and the output voltage of the converter 5 is input into an A/D converter 6 to be converted into digital values. When the digital value of the A/D converter 6 is '0', the vibration stop of the stylus 2A is detected.