The electronic structures of gas-phase exohedral holmium (Ho)-C-60 clusters, HonC60 with n=1-5, were studied by photoionization spectroscopy of the neutrals and photoelectron spectroscopy of their anions. Both spectroscopic analyses reveal that each Ho atom donates its three valence electrons to the unoccupied molecular orbitals of C-60. The electron filling through the charge transfer allows us to observe the unoccupied levels of C-60 due to high electron acceptability.