A regression mirror reflects light from a light projector in a reflecting direction opposite an incident direction. A light receiver combined with the light projector in a unitary unit receives the reflected light from the regression mirror. A detecting unit including an optical sensing unit having the light projector, the light receiver and the reflecting member is disposed so that a wafer is disposed in a moving path along which an optical path of the optical sensing unit moves relative to the wafer. An information-acquiring unit acquires position information of the detecting unit relative to the wafer, an arithmetic unit calculates mapping information about an arrangement of the wafer based on the position information and light-reception information provided by the light receiver.