An apparatus for detecting extraneous substances on a glass plate includes a first light projecting system arranged above a plane under examination on a glass plate, the surface of which is irradiated with an S-polarized laser beam at a first elevation angle, a second light projecting system arranged above the surface thereof which is irradiated with a P-polarized laser beam at a second elevation angle greater than the first elevation angle, and a light receiving system for receiving scattered light from the surface irradiated with the laser beams respectively emitted from the first and the second light projecting system at an elevation angle smaller than the first elevation angle. The light receiving system is arranged on a side opposite to the direction of irradiation with the normal line set up at the laser beam irradiation point therebetween and the output level of the P-polarized laser beam is set in specific relation to the S-polarized laser beam.