Applying in situ combination of angle-resolved photoelectron spectroscopy and k-resolved inverse photoemission to Si(100):Sb-1X1 and Si(100):Sb-2X1 surfaces, we have determined the surface band gaps between the Sb-derived filled and empty surface states at several points of the surface Brillouin zone. The values of the surface band gaps are compared with optical surface excitations obtained by means of surface differential reflectivity.