Apparatus for detecting extraneous substance on glass plate

An apparatus for detecting extraneous substances on a glass plate (1) comprises a first light projecting system (41) arranged above the glass plate surface under examination for irradiating the surface with an S-polarized laser beam at a first elevation angle, a second light projecting system (42) arranged above the surface for irradiating it with a P-polarized laser beam at a second elevation angle greater than the first elevation angle, and a light receiving system (5) for receiving light scattered from the surface at an elevation angle smaller than the first elevation angle. The light receiving system (5) is arranged on a side opposite to the direction of irradiation with a normal line set up at the beam irradiation point therebetween. The output level of the P-polarized beam is set in specific relation to the S-polarized beam.