PURPOSE: To detect a position of a mark at high precision even if an entire of a mark is not within a visual field of a camera.

CONSTITUTION: A mark part of a liquid crystal panel is photographed by a CCD camera and photographed image information is input to an image treatment device (S1). In the process, the mark is photographed so that at least a corner part of the mark is within a visual field of the CCD camera. An image treatment device detects a line Ly of an edge part which extends in a Y-axis (lengthwise) direction of a mark image from input photographed image information (S3 to S6). Furthermore, the device detects a line Lx of an edge part which extends in an X-axis (breadthwise) direction (S7 to S10), and obtains a position coordinate of an intersecting point S of the two lines Ly, Lx by operation (S11). Thereby, a position of a corner part of a mark is detected. The position of the corner part is compared with a standard position and deviation amount is obtained (S12).