PURPOSE: To stabilize a sensitivity level, by connecting a pair of infrared detection elements in parallel so as to make the polarities thereof inverse and alternately intermitting the incident energies of said two elements.

CONSTITUTION: Respective infrared detection elements 10-A, 10-B are arranged in parallel in inverse polarity and the outputs thereof are amplified by a low frequency amplifier and read by a spectrum analyser S. A. At this time, a shield plate 11 for alternately shielding the sensitive surface of two elements 10-A, 10-B is provided between an infrared generation source and a detection element and subjected to rectangular wave motion by an electromagnetic drive source 12.