PURPOSE: To perform a simple and reliable self-detection of good and bad, by detecting distribution of brightness of a double image of an optical image of a reference object, which has the same plane shape as that of a good object to be measured having a perforation and whose transmissivity of the perforation region is lower than that of other region, and the optical image of the object to be measured.

CONSTITUTION: A reference object 7 is irradiated through an illuminating device 10 and a reflecting mirror 12, an object 14 to be measured is irradiated through a light source 11 and a reflection mirror 13, and the two images are photographed by a TV camera through a prism group 16 to overlap them on a monitor TV19 to form a double image. The reference object 7 has the same shape as that of a good object 1, and is of a filmy object whose region corresponding to perforations 2a and 2b of the good object is blackened. The distribution of brightness of the double image is equalized through adjustment of an illuminating device 11 irradiating a good object 14. In the case of a poor object, a toroidal pattern is produced at the part wherein the perforations 2a and 2b are formed. This causes the good and the bad to be inspected in a simple and reliable manner.