Abstract: A flat light beam is projected from a light projecting unit to an object to be measured, sweeping its surface with a bright line. The image of the bright line is focused on pluralities of photosensor arrays and line photosensors. A plurality of positions in the bright line are computed by triangular surveying, based on the direction of projection of the flat signal light beam and the positions of those photosensors in each array from which the signal light component is obtained. The position of a gap in the surface of the object is computed, using the detected positions of the photosensors and a signal which indicates a dropout of the signal light component from the output of a line photosensor.