A semiconductor light position detector for a rangefinder which projects a light beam onto an object and receives a reflected light beam from the object to measure a distance to the object. A portion of the light-receiving surface of the detector on which the reflected beam will not form a light spot is partially covered with a member that blocks unwanted light. Thus, the malfunctions caused by the flare component of the light from the light projector and the malfunctions due to increases in the shot noise under a high illuminance are reduced greatly.