PURPOSE: To detect the end part of an object to be tested and to omit the positioning of the object to be tested, by completely synchronizing light rays having at least three kinds of wavelength, and scanning the object to be tested.

CONSTITUTION: A defect detecting scanning ray 11 having wavelength λ2 and end part detecting scanning rays 12a, 12b having wavelength λ1, λ3 are completely synchronized and irradiated to the surface of the object 20 to be tested. When the end part of the object 20 is inclined from the scanning direction, the rise and fall of respective detecting signals based upon irregularly reflected light with wavelength λ1, λ3 are shifted by T1, T2, T'1, T'2 each other at their timing. If T2=KT1 or T'2=KT'1 is satisfied, the defect detecting signals indicate the end part of the object 20. Since the end part can be detected and a gate signal is formed, the positioning of the object to be tested can be omitted.