When the sand surface layer is wet, the mass of sand moving is remarkably decreased compared with the case when it is dry, but a quantitative relation has not been clarified yet. Notwithstanding the importance of this problem for the research of coastal sand dune stabilization, in the conventional measuring methods of soil moisture content, it was attempted to take out the sample and to embed the measuring element in the sand or to make it contact with the sand surface, so that the conditions of blown sand and wind are influenced, making it impossible to carry out accurate experiment, retarding the progress of technique. Having noticed the fact that the color of sand surface changes by the moisture content of sand surface layer, the authors have been developing new methods of measuring optically the degree of color change, by which our object has been achieved. Here a simple method using an illuminometer on the market is mentioned. Namely, the intensities of illumination due to scattered reflected light on the sand surface in question and on the standard surface are measured, and the ratio R of the both intensities and the moisture content W of sand surface layer measured each time are employed for drawing calibration curves as shown in Figs. 6 and 7, then the value of W in the sand surface layer under consideration can be determined from the value of R independently of the environmental brightness.