PURPOSE: To make the device compact, light weight, and highly accurate, by adding a current for shifting the target by the diameter of an aperture to the correction data of the target at the time of the light servo, thereby shifting the target.

CONSTITUTION: A time division switch 3 is normally switched at three kinds of timings based on the relationship between the outputs of a horizontal axis coil and a vertical axis coil and a quantity of light. At the time of the light servo for detecting the quantity of the light, a shift voltage +α corresponding to the aperture diameter is added to the horizontal axis output and the vertical axis output, and the target is deviated from the aperture. When the deflecting current, to which the shift voltage +α is added, is applied to the horizontal axis coil H and the vertical axis coil V, the target is completely deviated from the aperture. The quantity of the light at this time is received by a photomultiplier. Said signal is appropriately processed, a high voltage circuit 16 is controlled, and the high voltage is varied by the increment of the variation in the quantity of the light. As a result, the fluctuation of the output data is prevented regardless of the quantity of the incident light.