The salt concentration dependence of the interparticle distance in colloidal crystals was precisely estimated by ultra-small-angle X-ray scattering for an aqueous dispersion of a poly(methyl methacrylate) latex. The nearest neighbor interparticle distance (2D(exp)) first increased with increasing coexisting salt concentration. After passing though a maximum, 2D(exp) decreased gradually. The maximum position was at the NaCl concentration where the number of NaCl molecules per particle divided by the analytical number of ionic charges on one latex particle was about 0.025, and the maximum value of 2D(exp) was almost equal to the average value calculated from the particle concentration, This is the first observation of such a curious behavior and no theory which has been proposed to date can explain this behavior.