1. A pattern measuring apparatus for determining coordinates of a scanning electron microscope (SEM) contour of a circuit pattern formed by transferring design data via a photomask, comprising: a storage for storing data on coordinates and luminance values of pixels of an image of the circuit pattern; an image contour acquiring unit that acquires a position of an image contour of the circuit pattern by scanning across a boundary between a region without the circuit pattern and a region with the circuit pattern to obtain the luminance values of the pixels of the image and determines a position of a pixel with a maximum luminance value as the position of the image contour; a shape structure classifying unit that classifies a contour shape of the design data into one or more shape structures characterizing the contour shape of the design data; a normal direction calculating unit that calculates, for each shape structure, normal directions to an approximation line obtained by approximating the shape structure with a smooth line; an arranging unit that arranges sections on the image contour in such a positional relation with each other as to form a same shape as the contour shape of the design data so that a circumferential ratio of each of the sections to the image contour is equal to a circumferential ratio of a corresponding one of the shape structures to the design data; and a SEM-contour position acquiring unit that acquires the coordinates of the SEM contour by scanning across the image contour in the normal directions calculated for the shape structures corresponding to the sections arranged on the image contour to acquire luminance values of pixels in the normal directions, and multiplies maximum luminance values of the acquired luminance values by a given factor to determine coordinates of pixels with luminance values thus obtained as the coordinates of the SEM contour.