PURPOSE: To increase the hit accuracy even in a short distance through acquisition of the tracking signal by providing the means to detect the spot size variation caused by approximation to the target and thus preventing the increment of the spot size.

CONSTITUTION: The following component elements are provided: band pass filter 9 which draws out the modulation component from the output of the preamplifier; modulation factor detection circuit 10 which delivers the output when the output level of filter 9 becomes lower than the set level; valve ON-OFF control circuit 11 which produces the valve switching signal via the output of circuit 10; valve switching mechanism 12 which performs the valve switching via the output of circuit 11; gas valve 13 which controls the outflow amount of the gas into the condenser system; and gas storing cylinder 14 which stores the gas to be flown out into the condenser system in order to convert the transmission factor of the infrared rays. Thus the gas is jetted out from the gas cylinder to perform the conversion of the transmission factor of the infrared rays within the condenser system in order to prevent lowering of the modulation factor.