Diffusion of iodide anion in rock pore water was studied in order to evaluated diffusivity of “non-sorbing” radionuclides in rockmatrix. Effective diffusion coefficients of iodide anion were measured, using a simple diffusion cell, on granite and tuff samples, which were selected as typical homogeneous rocks having different porosities. Effective diffusion coefficients of iodide anion were about 2×10^-12 m²/s for granite samples and about 7×10^-11 m²/s for tuff samples. The porosities, determined by a water saturation method, of the granite and the tuff samples were about 0.8% and 25%, respectively. The effective diffusion coefficient of iodide anion was much larger in the tuff sample than in the granite sample. This fact can be understood by the difference in their porosities.