A distance measuring device includes light a projector for projecting a beam of light onto an object at a distance to be measured, a light detector for detecting the part of the beam of light reflected from the object and outputting an output signal, an integrator for discharging or charging an integration capacitor corresponding to the output signal to integrate the output signal, an analog-to-digital converter for converting the voltage of the integration capacitor after a predetermined number of repetitions of a distance measuring routine, and a detector for detecting the distance to the object based on the converted signal, wherein the number of repetitions of the distance measuring routine is set to saturate the integration capacitor through the repetition, when the object is located at a short range alarm position.