PURPOSE: To expand a measurement range by arranging the photodetection surface of a photodetector at a position of the image point distance of the longest measurement distance from the center of a lens at right angles to the optical axis of the lens.

CONSTITUTION: A pencil beam 1 of 2mm in diameter emitted by a light source 6 of 20 milliwatt output strikes on an object 2 of distance measurement and is reflected irregularly. This irregularly reflected light 3 is condensed by the condenser lens 4 to illuminate the photodetecting element 5. Here, the element 5 is, for example, a CCD (charge-coupled device) and its photodetection surface is arranged at the image distance Kmax of the reflected light 3a from the object 2a of measurement at the longest measurement distance Lmax and at right angles to the optical axis of the lens 4 so that the image-formed reflected light illuminates the photodetection surface of the element 5. At the same time, the element 5 is so arranged as to project all of the reflected light 3b at the shortest measurement distance Lmin which is converged by the lens 4 on the other end of the photodetection surface of the element 5, and the image diameter of the reflected light projected on the element 5 at this time is maximum. Thus the measurement range is expanded.