A measuring apparatus is disclosed in which a surface to be measured in scanned by a scanning beam essentially normally incident upon the surface and the angular deviation of the beam of light regularly reflected from the surface is detected to measure the flatness of the surface. When the surface to be measured is such one which includes a low reflective part on which diffraction and/or scatter of light take place, and a high reflective part which may be considered almost to be a mirror surface, the measurement of flatness thereof is very difficult or impossible to carry out. This is applied, for example, to a semiconductor silicon wafer having thereon patterns for IC, LSI and the like. To enable to measure the flatness of such surface, the measuring apparatus according to the present invention comprises means for varying the cross sectional area of the scanning beam to the extent in which the cross sectional area of the scanning beam may cover the low reflective part and at least a portion of the high reflective part.