PURPOSE: To provide an optical system used for detecting a flaw, blister, dust, foreign matter, etc., of a liquid crystal display glass plate, a semiconductor wafer, a reticule or a mask glass blanks at high speed and at high S/N ratio.

CONSTITUTION: A laser is used as a light source, a telecentric scanning optical system is disposed, and an object to be, inspected is placed in the focal position of the optical system. A telecentric condenser lens 4 is disposed in such a manner that the focal plane is directed to the focal plane of a first telecentric scanning optical system and aligned therewith. In the rear side outgoing pupil of the condenser lens 4, zero-order diffracted light is missed by filtering, and only the scattered light is taken in. The telecentric scanning optical system and the telecentric condenser lens are combined together.