We have studied the effect of Lu substitution on the Kondo insulator YbB12 by high-resolution photoemission. Comparison of the spectra of YbB12, Yb0.5Lu0.5B12, and LuB12 reveals that the density of states (DOS) of the B sp-derived conduction band near the Fermi level is reduced in YbB12 over a rather wide (∼ 40 meV) energy region. Lu substitution (i) recovers the reduced B sp DOS, (ii) shifts the Yb 4f-derived Kondo peak towards higher binding energy, and (iii) decreases the Yb valence. These results are consistently analyzed using the Anderson-impurity model, and imply interaction between the Yb 4f ions mediated by the Yb 4f-B sp hybridization in YbB12.