PURPOSE: To decide a normal/defective state of marking in a marking time by detecting a reflected light from a material to be worked and recognizing a worked hole diameter through the amount of light quantity.

CONSTITUTION: A changeover mirror 2 is rotated 90° with respect to a machined hole 20 made by laser beam 8a for machining on the semiconductor wafer to be irradiated with laser beam 6a for measurement. The laser beam 6a is converged by a fixed aperture 7 in beam diameter to be smaller than the laser beam 8a and scans right and left through a galvanomirror 1. The laser beam 6a entering the surface of the semiconductor wafer 5 other than the drilled hole 20 is reflected to a power monitor 4 as normal reflected beam but since the laser beam 6a entering the drilled hole is diffused and turned into scattered light, the amount of light quantity reflected to a power monitor 4 is reduced. The diameter of the drilled hole 20 made by the laser beam 8a is determined by the amount of reflected light quantity to the power monitor 4 and the scanning width of the laser beam 6a, therefore, when the allowable value of the diameter of the drilled hole is preset, the normal/defective state of marking can be decided.