1. An alignment pattern detecting apparatus comprising an image pick-up device for scanning and picking up an optical image of a registration pattern formed on a wafer, the direction of the scanning being in a direction transverse to the lengthwise direction of the pattern, and means for symmetrically returning a video signal obtained from said image pick-up device to determine the degree of matching of said video signal with the video signal returned and for determining a return point associated with the best matching to detect said return point as the center of said alignment pattern.