PURPOSE: To perform infrared spectrometry while a region to be measured is observed by an optical microscope.

CONSTITUTION: During a period when no interferogram is sampled, reflecting mirrors 10, 18 are inserted in a light path and the visible light from a light source 8 is condensed by a transmission condensing mirror 4 to irradiate a sample and the transmitted light from the sample is condensed by a reflecting object mirror 6 to be guided to a video camera 24. During a period when an interferogram is sampled, the reflecting mirrors 10, 18 are shifted from the light path and the infrared rays guided from an infrared introducing port 26 are condensed by the transmission condensing mirror 4 to irradiate the sample and the transmitted infrared rays from the sample are condensed by the reflecting object mirror 6 and guided to an MCT detector 4 to be detected. The observation due to visible light and spectrometry due to infrared rays are alternately repeated.