An X-ray reflectance is measured with the use of an X-ray detector, which is not less than 10.sup.7 cps in upper-limit counting rate and is not more than twenty cps in noise level, under the condition that a measuring time length per interval of scattering angle 2.theta. is not more than fifty milliseconds, so that the measurement of one reflectance curve is completed in a short time as several seconds. In another aspect of the invention, the X-ray detector used is not less than 10.sup.7 cps in upper-limit counting rate and is not more than 0.01 cps in noise level, and the measuring time length per interval is not less than a hundred seconds, so that the X-ray reflectance curve is obtained with not less than a nine-digit dynamic range. The X-ray detector may be an avalanche photo diode.