The electron-density-functional method is applied to a jellium model for the surface-segregated binary (single-phase) solid solution. The segregation potential, thickness of the segregated layer, and the segregating component are determined and the following rule is derived for a binary (single-phase) solid solution of substitutional type: Atoms of the component which has the larger Wigner-Seitz radius segregate to the surface. The present theoretical predictions are completely compatible with those determined by Miedema's method and agree reasonably well with various experimental data.