PURPOSE: To quickly measure a drill bit by projecting energy to an object in continuous motion, detecting the reflected energy from the object, and determining the extended state of the object.

CONSTITUTION: A laser detection system 12 is arranged so that a laser beam 93 projected from a laser source 60 linearly reaches the tip of a drill bit 42 and the reflected laser beam enters a photodetector 70 via a light receiving lens 74 and an optical film 72. The laser beam 93 from the laser source 60 is projected to the drill bit 42, and the reflected laser beam is detected by the photodetector 70 and is fed to a computer via a cable 78. The computer analyzes the surface characteristic of the continuously moving drill bit 42 in response to the detected information, i.e., analyzes the surface characteristic by moving the drill bit 42 by the prescribed distance from the initially detected point. The computer judges the state in the longitudinal direction of the drill bit 42 based on the analyzed results at both points and judges the length of the region having the desired surface characteristic.