PURPOSE: To correctly attain range finding by positioning a photographic lens at the farthest lens set position, in case the incident light quantity is extremely small, ending forcibly the range finding before a prescribed range finding time elapses, in case the incident light quantity is extremely over.

CONSTITUTION: In case the incident light quantity is under, in case a voltage of an integral capacitor 28 is smaller than the smallest voltage V3, an output of a comparator 60 remains 'H', therefore, an output of and AND circuit 58 goes to 'H'. In such a case, a switching circuit 61 turns on, therefore, a prescribed voltage V4 is inputted to an encoder 33, the farthest lens set position signal is outputted from a decoding/latching circuit 35, and at the time of photographing, a photographic lens 38 is set to the farthest lens set position. Also, in case the incident light quantity is over, the voltage of the integral capacitor 28 reaches the maximum voltage V2, before a standard distance measurement time elapses, there fore, an output of a comparator 59 goes to 'H', light emission of a light emitting diode 10 stops, and the range finding is stopped before the integral capacitor 28 is saturated.