PURPOSE: To increase the hit accuracy through acquisition of the tracking signal even in a short distance by providing the means to attenuate the transmission factor of the infrared rays for the condenser system and thus preventing the early saturation of the preamplifier.

CONSTITUTION: The following component elements are provided: clamp circuit 9 which delivers the output in case the output level of the preamplifier exceeds the set level; valve ON-OFF control circuit 10 which produces the valve switching signal via the output of circuit 9; valve switching mechanism 11 which opens and closes the valve via the output of circuit 10; gas valve 12 which controls the gas flow amount to the condenser system via mechanism 11; and gas storing cylinder 13 which stores the gas flowing out into the condenser system to attenuate the transmission factor of the infrared rays. Mechanism 11 is actuated by circuit 10 to control the valve, and thus the gas is jetted out within the condenser system to attenuate the transmission factor of the infrared rays. Thus the saturation time is delayed for the preamplifier.