A rangefinder apparatus includes an infrared light-emitting diode for projecting light toward a range-finding object; a position sensitive device for detecting projected light reflected from the range-finding object and outputting a signal according to a position at which the reflected light is detected; and arithmetic circuit for carrying out an arithmetic operation according to the signal from the photosensitive device and outputting a signal corresponding to a distance to the range-finding object; an integrating circuit for integrating the signal from the arithmetic circuit by repeatedly discharging an integrating capacitor with an integrating period in response to the signal from the arithmetic circuit, to output a signal corresponding to a result of the integration produced by charging the integrating capacitor; a CPU for detecting the distance to the range-finding object according to the signal from the integrating circuit; a battery for supplying a power source voltage; and a step-up regulator for raising the power source voltage of the battery through an oscillating action and outputting the raised voltage to the position sensitive device, arithmetic circuit, and the like, wherein the integrating period of the integrating circuit is prevented from being a constant period.