PURPOSE: To suppress the affection of a background remarkably upon tracking a target by a method wherein the adding range of an output, received by a detector and converted into an electric signal, is switched to the range of visual sight in accordance with the condition of catching or tracking of the target.

CONSTITUTION: A detector 2 is provided with a plurality of detecting elements and is arranged so as to provide light receiving function equivalent to a traditional reticle. Light, entered such a detector 2, is converted by respective detecting elements into an electric signal and is supplied to the amplifier 3 of the detector. The amplifier 3 of detector amplifies the inputs independently respectively and provides an adding circuit 4 with the inputs. The adding circuit 4 determines the adding range of the outputs of the amplifier 3 of detector by the output of a target catching/tracking and deciding circuit 5. The target catching/tracking and deciding circuit 5 discriminates whether an output condition is target catching condition or target tracking condition automatically based on the output condition of the adding circuit 4 and, then, inputs the result of the discrimination into the adding circuit 4. In this case, the adding circuit 4 is controlled so that the outputs, corresponding to all detecting elements, are added upon catching the target while the outputs of detectors, corresponding to the detecting elements necessary for target tracking, are added upon tracking the target and the sight of view can be switched into wide visual sight upon target catching but can be switched into narrow visual sight upon tracking the target automatically.