To detect fouling of a light projecting/receiving window highly accurately by a simple constitution, in an optical range finder for measuring a distance to a measuring object by projecting light toward the measuring object and by receiving reflected light from the measuring object through the light projecting/receiving window through which light is transmitted.

In a fouling detection part 107 for inputting a ranging result by a distance value calculation part 101 and a received-light amount level, when acquiring a ranged value equivalent to a distance to the light projecting/receiving window 6, it is determined that fouling adheres to a domain of the light projecting/receiving window 6 corresponding to a measuring point, and fouling intensity is detected from the received-light amount level in measurement. Then, a fouling level is determined from the measuring point determined that fouling adheres thereto and the fouling intensity at the measuring point, and an alarm device 108 and a removal device 109 for removing fouling on the light projecting/receiving window are operated based on the fouling level, and range finding operation such as change of a measuring point position/light projection intensity/light reception sensitivity is corrected.