The light source is a low-coherence light source, and the optical path is regulated so that the reflected light from the upper surface of each metal wire and that from the reference surface interfere with each other during the first measuring operation, and so that the reflected light from the small regional surface of the conductive layer and that from the reference surface interfere with each other during the second measuring operation. The light detecting surface is divided into a plurality of detecting unit regions, and obtains interference fringe information corresponding to the upper surfaces of the metal wires or the general shape of the first or second virtual contact surface engaged with the small regional surfaces of the conductive layer on the basis of the light intensity averaged and detected with respect to the detecting unit regions.