Abstract: In a semiconductor fabricating apparatus having a wafer rotational processing device provided with a wafer chuck, a wafer centering mechanism comprising: a plurality of movable stops disposed with even angular spacing around the center of the chuck and being capable of being moved radially toward and away from that center thereby to clamp the periphery of a wafer conveyed onto the chuck and subsequently to release the same; a plurality of pairs of wafer position sensors disposed at spaced-apart positions in a single row along the path of advance of the wafer, the sensors of each pair being disposed on opposite sides of and equidistant from the chuck center, each sensor operating to detect whether or not a part of the wafer exists thereabove and to generate a corresponding output signal; and a stop moving mechanism for moving each of the movable stops in the manner stated.