The decrease in the light reflection of ordinary window glass plates by marring them with falling carborundum powder was measured. Effects of grain size, falling rate, and falling height were deter-mined by using carborundum powders BC#40, #60, #80, #100 and GC#120 for the amounts ranging from 50g. to 2kg. and for the falling heights of 0.5, 1.0, and 1.5m. Apparatus for measuring the reflectivity and the method of its calibration by the standard optical wedge are described. Results are as follows: (1) The reflection of light decreases as the grain size becomes larger, as the falling rate becomes faster, and as the falling height becomes higher, and approaches to the saturation value which is different for each case. (2) For smaller grain size and for lower rate of fall of the carborundum powder, the reflectivity vs. grain size or rate of fall curves are not smooth, and the reflectivity rises step-wise with the increase of brain size or the rate of fall.