An autofocusing circuit includes an extrinsic light eliminator including a light emitter such as an infrared emitting diode for projecting a signal light reflected by an object to be photographed and detected by a photodetector to determine the distance to the object. A chopper integrating circuit to convert the photocurrent from the detector to a corresponding voltage and simultaneously amplifies this voltage. In a preferred embodiment, the operating period of the chopper integrating circuit is set to 0.025 sec. multiplied by an integer in order to assure that any extrinsic light components derived from a commercial power supply is reliably eliminated from the detected photocurrent no matter whether the commercial power supply is of 50 Hz or 60 Hz.