PURPOSE: To make operation easy by making the teaching operation unnecessary by optically comparing the plate to be tested directly with the standard plate.

CONSTITUTION: Light passed through a through-hole 3a of a reference plate 2a from a light source 1 is reflected by a half mirror 7 and is arrived in a light sensor 8a. As a result, high level output signal is generated in the sensor 8a. A part of light transmitted from the light source 1 through the half mirror 7 reaches a test plate 2. In case when a through-hole 3 is there, it reaches the light sensor 8a through the through-hole 3 and high level output signal is generated in the light sensor 8a. In case when a through-hole 3 is not existed, output signal of the light sensor 8a is low level. Therefore, if through-hole 3 is worked in accordance with the through- hole 3a of a plate 2a which is the reference plate for the plate 2 to be tested or not can be discriminated by comparing the output signal of the light sensor 8a and the output signal of the light sensor 8b with the use of a comparator 9.