A pattern having identification features on measurement lines of a multi-slit is projected to an object, and an image is acquired. Based on positions of identification features detected from the image, a measurement line number is identified for each identification feature. Based on a position of a selected identification feature on the image, a position and a slope of an epipolar line in a coordinate system of the pattern are calculated. The position and slope of the epipolar line are calculated by projecting a straight line extending from the position of the selected identification feature to a line-of-sight direction onto the coordinate system. Because the position of the selected identification feature in the coordinate system is on the epipolar line, a measurement line having an identification feature on the epipolar line is searched on the pattern to identify the measurement line number for the selected identification feature.