It is known that the doped Cu suppresses Al-electromigration caused by high density electric current in VLSI interconnects. We investigate the mechanism of suppression with the method of quantum mechanical calculation. We introduce a periodic surface model formed by three Al layers with a Cu or an Al atom approaching the surface and calculate the total energies, charge distributions and Mulliken charges of the Cu and Al atoms varying the positions of the adatoms and the orientation of the surface. Our calculation shows that the adhering Cu atom draws electrons out of the Al atoms around itself and gets more negative charge than the adhering Al atom, Moreover, we found that the Cu atom adhered closer to the surface than Al and that the energy barrier against the Cu atom moving on the surface was higher than that against Al. It is expected that these features of the Cu atom in Al crystal play an important role in the suppression of Al-electromigration by Cu. In addition, we found that the barrier against the motion along the surface and the stable location of the adhering atom highly depend on the orientation of the surface.