A distance detecting device comprising radiation projecting means for projecting a radiation beam; radiation sensing means which is disposed away from the radiation projecting means with a given length and is arranged to receive a reflected radiation beam coming from an object whose distance is to be detected as the radiation beam is projected from the projecting means; and movable means for relatively varying the incident condition of the projected radiation beam onto the sensing means. The distance to the object is found either by the position of the movable means or by the amount of displacement thereof which occurs at a time point at which the sensing means optimally receive the reflected radiation beam coming from the object in the process of relatively varying the incident condition of the projected radiation beam onto the sensing means. In the incident condition varying process, the output signal of the sensing means and a delayed signal which is obtained by delaying the output signal a predetermined period of time are compared with each other to detect a time point at which the output signal of the sensing means has reached its peak level and, accordingly, to detect when the sensing means optimally receive the reflected radiation beam coming from the object.