PURPOSE: To detect the damage of an object in eary period by comparing a digitalized signal indicating the energy reflected by a surface of the object with a threshold value for detecting a state of the object in a sensor for detecting the state of the object during the continuous reciprocating motion of a drill bit or the like.

CONSTITUTION: In a drilling machine for drilling a circuit board, the circuit board positioned on a servomotor driven work table is drilled by a drill bit discending and rotating at a high speed. A non-contact optical detecting system is mounted for monitoring a state of this drill bit 42, the laser is irradiated to a point of the drill bit 42 by a laser detecting sensor 12, and a first signal indicating the reflected energy is digitalized, and then compared with a first threshold value. A second signal indicating the energy reflected from the drill bit 42 and amplified, is digitalized, and compared with a second threshold value, and the state of the drill bit 42 is judged on the basis of the results of comparison.