PURPOSE: To suppress the irradiated surface illuminance of a target below a specidic level by controlling the beam spread angle of a light transmission part so that the level of a signal from the target is within a specific range.

CONSTITUTION: The light transmission part 1 consists of a light source 8, zoom type telescope 9, and its driving part 10, reflected light from the target (not shown in a figure) is converted photoelectrically by a photodetection part 2; and a tracking signal processing part 3 calculates the deviation from a reference position, and a servo rack driving part 4 is controlled according to the calculated value to drive a servo rack 5 with which photodetection parts 1 and 2 are coupled. On the other hand, the signal level of the target is compared with a set level by a target level comparison part 7 with the output signal of the photodetection part 2, and the driving part 10 is controlled with th comparison result signal to adjust the spread angle of light from the light source 8.