Method and device for detecting micro foreign matter within transparent plate

A light-blocking micro foreign matter within a transparent plate, if any, is stably detected and information on the depth of the micro foreign matter is acquired. A method is used for detecting a micro foreign matter (4), if any, present in a transparent plate (1) having a uniform index of refraction and a specific thickness by measuring bright spots (E, F) produced by scattering or the like of the projected light beam (2). The method is characterized in that the light beam (2) is made to enter the inside of the transparent plate at a predetermined angle of inclination with respect to the thickness direction of the transparent plate, thereby an inside light beam (I) entering the inside and an inside light beam (II) of when the inside light beam (I) is reflected by the surface of the transparent plate are produced inside the transparent plate, the transparent plate and the light beam (2) are relatively moved (in a direction G) without varying the entering angle of the light beam, the bright spots (E, F) produc