The valence-band interaction at the polycrystalline Al-Sn interface was investigated by ultraviolet photoemission spectroscopy. Al thin films were deposited on the Sn polycrystalline thick film by the vacuum evaporation technique. Only at small thicknesses of the Al film (less than 1.0 nm), two decreased and two increased regions of intensity were recognized in ultraviolet photoemission spectra. This phenomenon was explained by assuming that valence-band interorbital interaction between Al and Sn formed bonding and antibonding orbitals (bands) below the Fermi level. The interorbital interaction was first observed in s, p metal systems, although it has been reported in d-metal systems such as Au-Ru and Cu-Ru. The three systems, Al-Sn, Au-Ru and Cu-Ru, have a common nature in the thermodynamic phase relation; that is, they do not easily form a solid solution but form a mixture of two solid phases at room temperature. These experimental data suggest the hypothesis that the repulsive interaction between two elements in eutectic binary systems comes from such interorbital interaction.