PURPOSE: To discriminate whereabout, the surface or the back side, of defect and make it possible to ascertain the quality on the separate basis by detecting the defect in the comparison of the spot image in the case of no defect with the spectrum spot dispersing in the range of wide angle in the case of existing the defect.

CONSTITUTION: The laser beam from the laser light source is swept in the X-axial direction through a vibration mirror 3 to be further converged into a spot on the surface of the panel plate 5 through a projecting lens 4. Toward the Y-axial direction the scanning of the whole region is completed through the travelling of the panel plate 5. The reflected ray 2, namely the regular reflecting ray of the projection beam 2' is added to the photoelectric transducer 7 through a light receiving lens 6 after the spot image shielded by a mask 13. Simillarly on the back side of the panel plate 5 the transmission light is detected through the mask 13', light receiving lens 6' and the photoelectronic transducer 7' and through the output of the photoelectronic transducers 7 and 7' the discrimination of the surface or the back side is performed and besides the defect is detected through the separate criterion toward the surface or the back side. Thereby the defect on the back side having little influence on the quality of a product can be some extent neglected.