In order to fabricate an X-ray mirror telescope the roughness of mirror surfaces was investigated using X-ray scattering. The scattering profiles of plate glass were measured with Al-K (8.34-A) X-rays. The spectral structure of surface roughness was revealed by the angular distribution of the X-ray scattering. The power-spectral density functions of the surface-height distribution for these materials (except for a gold evaporated surface) were represented by power-law spectra with power indices ranging from -1 to -2. The rms heights were derived to be 1.8-8.3 A for a wavelength range as expected from the power-law spectrum. The results obtained with this method were found to be consistent with those with an ordinary optical profilometer.