We present a molecular theory for the friction coefficient of an ion based on the interaction-site model for molecular liquids and on the mode-coupling theory. The ionic friction is describe in terms of the response of collective excitations in solvent to a solute displacement, and the resultant formula expresses the friction as an amount of the energy dissipated during the relaxational processes after the solute perturbation. Utilizing this viewpoint, the ionic friction is shown to be naturally decomposed into hydrodynamic and dielectric contributions and their coupling. it is demonstrated from theoretical calculations of the ionic frictions that both classical pictures,the solventberg and dielectric friction pictures, are responsible for large frictions of small ions, and that the coupling term, which has been disregarded in theoretical considerations so far, should also be properly taken into account in dealing with small ions.