PURPOSE: To accurately perform automatic focusing even in an infrared microscope having an aperture for restricting visual field by constituting the microscope so that light from a sample is allowed to pass only a 2nd optical path to perform the automatic focusing in an automatic focusing mode and the light from the sample is allowed to pass only a 1st optical path or the 1st and the 2nd optical paths in a measuring mode.

CONSTITUTION: The aperture 23 for restricting the visual field is provided only in the 1st optical path between the 1st and the 2nd optical paths branched by a 1st mirror 21. In the automatic focusing mode, a CPU allows the 1st mirror 21 to get in the optical path and a 4th mirror 25 to retract from the optical path. Thus, the light from the sample 19 advances in a path (2nd optical path) through the 1st mirror 21, the 2nd mirror 22 and a tri-mirror head 15, and gets in the video camera of the mirror head 15 without being restricted by the aperture 23. In the measuring mode, the CPU allows the 1st mirror 21 to retract from the optical path and the 4th mirror 25 to get in the optical path, so that only the image restricted by the aperture 23 gets in the mirror head 15.