PURPOSE: To quickly detect the presence of a drill bit near a drilled surface without retreating the drill bit from the surface by detecting reflected energy intermittently reflected at a frequency which is directly connected with the frequency of continuous repetitive operations.

CONSTITUTION: The light from a laser diode 60 is emitted to a drill bit 42 through a strip radiation optics 64. The convergent laser light is reflected from the surfaces of the two recessed spiral grooves of the bit 42 and condensed through a receiving lens 74. The condensed light beams are collected to a solid- state photodetector 70 after the light beams are collimated and filtrated through an optical filter 72. Thus the laser beam is reflected to the photodetector 70 from the surfaces of the two spiral grooves of the bit 42 and gives a transmitting signal having a frequency which is directly connected with the rotating frequency of the bit 42. Therefore, the presence of the oscillation signal of the laser light at a predetermined frequency is detected.