A new theory of a binary solvent is developed to study the effects of the density of solvent particles around a large solute particle on friction. To develop the theory, the solvent particles are assumed to be much smaller than the solute particle, and then a perturbation expansion is employed. The expansion allows one to derive hydrodynamic equations with boundary conditions on the surface of a solute. The boundary conditions are calculated from the radial distribution functions of a binary solvent. The hydrodynamic equations with the boundary conditions provide an analytical expression for the friction. The developed theory is applied to a binary hard-sphere system. The present theory shows that the friction in the system has larger values than those predicted by the Stokes law.