PURPOSE: To realize highly accurate centering of a wafer by measuring the positional shift between the centers of the wafer and a rotary stage and the position of orientation flat based on the signals from an angle sensor and a CCD linear sensor.

CONSTITUTION: A wafer 1 is sucked to a wafer chuck 2 fixed to a rotary stage 3 also fixed with an angle detector 4. A light projecting section comprises an LED array 9 and a CCD linear sensor 10 is disposed on the water surface closely to the under side thereof. An operation control section 12 measures the positional shift between the centers of the wafer and the rotary stage, as well as the position of orientation flat, using the single CCD linear sensor 10 so that the positional shift can be corrected and the orientation flat can be positioned. This structure can decrease the number of components and since no error is produced theoretically, highly accurate centering of wafer can be realized.