A thermal image detecting system includes a moving part having pyroelectric thermal detection element, a chopper for opening/closing a path of infrared rays and a chopper temperature detection element, a rotating section for rotating the moving part, a drive control section for driving the chopper and the rotating section, a band amplifier for amplifying a detection signal from the pyroelectric thermal detection element, a timing output section for outputting a signal synchronous with an opening/closing operation of the chopper, a peak detection section for successively holding the maximum value and the minimum value of a signal from the band amplifier, and an operational processing section for performing operational processing on a thermal image signal on the basis of a difference between the maximum value and the minimum value or a difference between a reference signal from the band amplifier in a closed state of the chopper and the maximum value, or the minimum value and the temperature of the chopper.