  Insulator-to-metal transition in Ca$_{2}$RuO$_{4}$ has drawn keen attention
because of its sensitivity to various stimulation and its potential
controllability. Here, we report a direct observation of Fermi surface, which
emerges upon introducing excess oxygen into an insulating Ca$_{2}$RuO$_{4}$, by
using angle-resolved photoemission spectroscopy. Comparison between energy
distribution curves shows that the Mott insulating gap is closed by eV-scale
spectral-weight transfer with excess oxygen. Momentum-space mapping exhibits
two square-shaped sheets of the Fermi surface. One is a hole-like $\alpha$
sheet around the corner of a tetragonal Brillouin zone, and the other is an
electron-like $\beta$ sheet around the $\Gamma$ point. The electron occupancies
of the $\alpha$ and $\beta$ bands are determined to be $n_{\alpha}=1.6$ and
$n_{\beta}=0.6$, respectively. Our result indicates that the insulator-to-metal
transition occurs selectively in $d_{xz}$ and $d_{yz}$ bands and not yet in
$d_{xy}$ band. This orbital selectivity is most likely explained in terms of
the energy level of $d_{xy}$, which is deeper for Ca$_{2}$RuO$_{4+\delta}$ than
for Ca$_{1.8}$Sr$_{0.2}$RuO$_{4}$. Consequently, we found substantial
differences from the Fermi surface of other ruthenates, shedding light on a
unique role of excess oxygen among the metallization methods of
Ca$_{2}$RuO$_{4}$.
