  Carbon- and boron-2$p$ states of superconducting and non-superconducting
boron-doped diamond samples are measured using soft X-ray emission and
absorption spectroscopy. For the superconducting sample, a large density of
hole states is observed in the valence band in addition to the states in the
impurity band. The hole states in the valence band is located at about 1.3 eV
below the valence band maximum regardless of the doping level, which cannot be
interpreted within a simple rigid band model. Present experimental results,
combined with the first principles calculations, suggest that superconductivity
is to be attributed to the holes in the valence band.
