PURPOSE: To safely, easily and accurately detect a variation in an optical property due to fog of a transmitting window material, adherence of dust by introducing a laser light from a reference laser to the material, and detecting a reflected light, transmitted light or a scattered light through the material.

CONSTITUTION: A laser light from a reference laser 1 impinges obliquely to a transmitting window material 5a (5b) through a prism mirror 2, a mirror 3a (3b), a half mirror 13a (13b), and a reflected light from the material 5a (5b) is detected by a photodetector 8a (8b) through an interference filter 12a (12b). In this case, if the material 5a (5b) has fog or dust, the intensity of the detected light becomes small. Accordingly, an output signal ratio from a monitoring photodetector 14a (14b) is obtained by a discriminator 9. Then, it is compared with a threshold output ratio previously determined here, its result is displayed on a display unit 10 to know a variation in an optical property. Here, when the position of the photodetector 8a (8b) is deviated, a scattered light, etc., can be detected.