A distance measuring device comprising a flood-light projecting means having a flashing device adapted to direct a flashlight to an object to be photographed, a light receiving means incorporating a light slide rheostat disposed at a predetermined distance from said projecting means and a lens adapted to allow light rays reflected from said object to impinge, in the form of a beam, on the light receiving surface of said rheostat, a converting means for converting the variation of the resistance amount of said rheostat into the variation of voltage and an indicating means for indicating said voltage variation, particularly useful in photographic cameras. Also, an automatic focusing device having the same constructional features as the aforesaid distance measuring device, except that it has an objective lens adjusting means operated by said voltage variation instead of said indicating means.