1. A method for deciding an image data reduction ratio with which image data is reduced in pattern model positioning in image processing where an object to be searched that is similar to a pre-registered image is searched to be positioned out of an image to be searched by use of a pattern model corresponding to the registered image, the method including the steps of: computing a sharpness of each edge point based on the edge points with reference to an edge image where the edge points are extracted from image date; and deciding an image data reduction ratio based on each edge sharpness such that positional accuracy of the edge points becomes higher than a predetermined value.