The excess magnetic moment due to an impurity atom substituted in ferromagnetic Ni is calculated by the Koster-Slater theory. (The impurity atoms are Co, Fe, Mn, Cr and V.) The degeneracy of the five d-bands is taken into account, but the s-d interaction is not. In place of the true Wannier function, the orthogonalized 3d-atomic orbitals are adopted. We start with the real band structure of the ferromagnetic nickel and construct the Green's functions for the impurity center and its nearest neighbour sites. The impurity potential is constructed as spin-dependent and assumed to extend to the nearest neighbour sites. The results of the calculation show that the positive excess magnetic moment appears for Co, Fe and Mn impurities, but the negative moment for Cr and V. The excess moment is confined to the impurity center in the case of Co, Fe and Mn, but it is extended to the nearest neighbours in Cr and V. Thus, we see that the theoretical results are in reasonable agreement with experiment.