Abstract: To precisely measure bead cutting shapes of electric resistance welded pipes without being affected by difference in luminance level between cut and uncut portions in optical cutting images, an image is obtained by overlaying an optical cutting image with the optical cutting image subjected to thinning processing. A profile of the welded pipe is approximated with a quadratic function and a region containing the bead apex coordinates is identified as the bead. Shape data of the pipe surface at the portion corresponding to the bead portion is obtained from the preset left and right boundaries of the bead portion and the apex position of the separately-calculated bead portion, and bead width, height, slope angle, and unevenness at the left and right boundaries between the bead portion and base pipe portion, are each calculated, based on the left and right bead shape approximation functions and base pipe shape approximation function.