The light scattered out of multimode optical waveguides formed by ion exchange from molten salts of silver nitrate was measured in order to clarify the mechanism producing scattering loss. The angular distributions of the scattered light can be explained by the loss originating not only from surface roughness but also from silver ion concentration diffused in the glass. For a relatively thick waveguide of diffusion depth greater than 4.5 &mgr;m which was obtained by diffusion time longer than 7 h at 300 °C, it was found that the effect of bulk inhomogeneities is predominant in the air radiation. The correlation length in the direction of beam propogation was estimated to be the order of a wavelength of light.