In a method for X-ray reflectance measurement in which an intensity of a reflected X-ray is observed for each incident angle, a measuring scale for the incident angle .omega. is corrected, before the reflectance measurement, using an analyzer crystal. In the corrective operation, the aperture width of the receiving slit is made wider than in the X-ray reflectance measurement, and the analyzer crystal is inserted in the reflection path, and then the reflected X-ray intensity is detected. In this condition, the incident angle .omega. of the incident X-ray to the sample surface can be determined accurately, and thus the measuring scale for the incident angle can be corrected. Thereafter, the analyzer crystal is removed from the reflection path, and the X-ray reflectance measurement for the sample surface is carried out.