1. A computer implemented method of recognizing a position of an object, comprising the steps of: picking up an image of an object having a circular arc shape as at least part of its profile using an image pickup device; converting image information of the picked-up image into gray image data and storing the gray image data in a storage device; scanning the stored gray image data with a profile scanning window composed of a plurality of gray-level detection areas crossing an imaginary scanning circular profile corresponding to the circular arc shape on the gray image and arranged circularly along the imaginary scanning circular profile and using it to measure a gray-level representative value of an inner-side gray-level detection area and a gray-level representative value of an outer-side gray-level detection area which are inside and outside of the scanning circular profile; calculating, at each scanning position during scanning and for each gray-level detection area, an absolute value of a difference between the measured gray-level representative value of the inner-side gray-level detection area and the measured gray-level representative value of the outer-side gray-level detection area; determining a number of gray-level detection areas in which the absolute value of the difference is larger than a specified gray-level threshold value; detecting a position of the profile scanning window where the calculated number of gray-level detection areas becomes a maximum with respect to their neighborhood, as a position of the circular arc shape of the object; and recognizing the position of the object from the detected position of the circular arc shape thereof.