The present invention includes steps of, determining a search starting position; setting a center position, and a first position and a second position with a distance provided therebetween; obtaining a measurement point group including measurement points of the center position, the first position, and the second position; and determining a measurement point closest to the tip portion in the measurement point group based on positions of the measurement points in a second direction and selecting a position of the measurement point in a first direction as a selected position. When the measurement point group is obtained initially after the determination of the search starting position, the search starting position is set as the center position; and when the measurement point group is obtained after the selection of the selected position, the selected position is set as the center position. The distance is narrowed for every selection of the selected position.