This paper describes an optical detecting method of the edge position for measuring the profile and the dimensions of an object with a sharp edge. The measuring principle is based on the straightness of a laser besm and its diffraction properties at the edge of various shapes. The laser beam of Gaussian distribution is scanned parallel to the beam axis. When the laser beam is intercepted with a sharp edge at right angles, the transmitted beam power is reduced from the original total power according to the relative position of the edge to the beam axis. The detecting accuracy using a straight edge and a circular edge is analyzed and expressed by numerical solutions. A fundamental detecting apparatus for the edge position based on the measuring principle is presented and the basic performance of this apparatus is investigated. The results show that the detecting method proposed is appropriate for the noncontact detection of the edge position.