In a preparation process, one workpiece W is photographed from a plurality of angles, the photographed image is segmented into patches to obtain a plurality of sample patch images, the sample patch images are classified, and each sample patch image is stored while being corresponded with a position information to a reference position of the workpiece W. Image recognition process obtains a reference position of the workpiece W in a patch image, by photographing piled-up workpieces W, segment the photographed image into patches to obtain a plurality of patch images, classify the patch images in same classification method of the sample patch images, accumulating a position information to the reference position of the workpiece W, which is stored while corresponded to the sample patch image of the same classification as the patch image for each patch image, and on the basis of the accumulated result.