We report here a multiple-reflection type ultra-small-angle x-ray-scattering apparatus constructed according to the principle proposed by Bonse and Hart. The ultra-small-angle x-ray scattering technique enables us to investigate the distribution of solutes or particles in turbid systems, which could not be studied by the conventional light-scattering and microscopic methods. By this method, the density fluctuation from an order of several thousand angstroms to several micrometers is now detectable. In this paper, the ultra-small-angle x-ray scattering apparatus was calibrated by using well-defined latex particles.