PURPOSE: To enable a plate to be positioned without fail while avoiding such troubles as the wafer pollution and the plate manufacturing cost up.

CONSTITUTION: The periphery of the plate 4 is irradiated with the parallel beams C making an angle of about 30° with the tangent A of the plate 4 from a light source 10 so as to measure the reflected rays B in the radial direction perpendicular to the tangent A using a photosensor 11. When an positioning trench traverses the front part photosensor 11, the shadow of the parallel beams C being obliquely irradiated from the light source 11 is to be cast on the positioning trench so as to reduce the incident light quantity of the photosensor 11 thereby enabling the passage of the positioning trench to be detected.