PURPOSE: To stably trace and decide infrared targets in both a highly sensitive area and a low luminance area by providing a function to very the amplification factor of a detecting element amplifier, and making infrared signal receiving sensitivity into high sensitivity extending over a wide area.

CONSTITUTION: An infrared signal scanned by an optical system 2 is received with a detecting element 3, and is carried via a signal line 8. A reception signal is switched by an external change-over switch, and is amplified by the detecting element amplifier 4 with the amplification factor controlled by a switch circuit 7, and is processed by a signal processor 5. The switch circuit 10 of the signal processor 5 selects a trace decision circuit 11 in an ordinary operation, and a comparator 12 and an arithmetic circuit 13 in the trace decision circuit 11 perform trace decision processing corresponding to an input signal level. When a result enabling trace to be performed is obtained at the trace decision circuit 11, trace processing is performed by switching the operation to a trace processing circuit 14.