  We have investigated the low energy electronic state of a boron-doped diamond
thin film by the ultrahigh resolution laser-excited photoemission spectroscopy.
We observed a clear shift of the leading edge below 11 K indicative of a
superconducting gap opening ($\Delta \sim 0.78$ meV at 4.5 K). The gap feature
is significantly broad and the well-defined quasiparticle peak is not
recognizable even at the lowest temperature of measurement 4.5 K. We discuss
our result in terms of possible disorder effect on superconductivity in this
system.
