1. An image data compression method for compressing data on an image to be searched in pattern model positioning in image processing of searching out of the image to be searched and positioning an object to be searched that is similar to a pre-registered image by use of a pattern model corresponding to the registered image, the method including the steps of: computing an edge angle image including edge angle information with respect to each pixel constituting an image; transforming an edge angle of each pixel into an edge angle bit image expressed by an edge angle bit indicating an angle with a pre-defined fixed width; and performing an OR operation on an edge angle bit of every pixel included in an OR operation region decided in accordance with a reduction ratio for reducing the edge angle bit image in order to create an edge angle bit reduced image reduced from the edge angle bit image, to create an edge angle bit reduced image made up of reduced edge angle bit data representing each OR operation region.