According to the invention, each detecting element pair of a wafer detector is constituted by light-emitting and light-receiving elements of transmission-type and is arranged at a position corresponding to a fitting edge portion of one of a plurality of wafers. The number of detecting element pairs is equal to the number of wafers which can be stored in the wafer counter. Light emitted from a light-emitting element to a light-receiving element is kept substantially perpendicular to a surface of a wafer and is shielded when a wafer is engaged in a wafer counter. A change in the amount of light received by the light-receiving element is detected by a signal processor electrically connected to each of the detecting element pairs. The positions and number of a large number of wafers can be accurately and instantaneously detected with an extremely simple structure.
