In this paper, an experimental set up and a method of measuring the scattering angle are introduced. The measuring method of the scattering angle is as follows. The guided light wave propagating in the waveguide is coupled out by a prism. The well known m-line can be observed. By measuring the distribution of the light intensity along the m-line, the line width which covers a region of -40 db from the peak of the curve can be read out. While converting it into an angle of slope of the light beam, the scattering angle is obtained. It has been found that the influence of the scattering on the profile of the to—line is not considerable in the region from 0 to -20 db. The segment of the curve coincides with the intensity distribution of the incident beam. On the oiiher hand, the intensity distribution of the incident beam can be considered as a Gaussian one. Thus we can take the segment from 0 to -20 db of the profile of the m-line as the intensity distribution of the incident beam and calculate its width at the -40 db region. Considering the intensity distribution of the incident beam, we redefine the scattering angle. Thus, the error of obtained result is reasonable.