PURPOSE: To detect a distortion in a semiconductor wafer so as to prevent the occurrence of defocusing and measurement errors caused by the distortion by providing numerous through holes through a stage main body from its semiconductor wafer placing face and a light reflecting type distance measuring instrument to the through holes.

CONSTITUTION: Optical fibers 12 are inserted into through holes 13 formed through a stage main body 3 in vertical direction from the wafer placing face 3a of the main body 3. When a wafer 1 is attracted by suction to the main body 3 in a distorted state due to a foreign matter 8 between the wafer 1 and face 3a, the quantity of the reflecting light from the wafer 1 made incident to a light receiving section 15 through the fiber 12 facing the distorted part increases. A light reflecting type distance measuring instrument main body 11 detects the increase in light quantity and, when the distance from the rear surface of the wafer 1 to the leading end of the optical fiber 12 is larger than a specific value, generates an alarm. When this stage is used, therefore, the distortion in the wafer 1 can be detected and the foreign matter 8 can be removed before the wafer 1 is subjected to an exposing process, etc.