  CeCoSi with no local inversion symmetric crystal structure ($P4/nmm$)
exhibits a phase transition of unknown origin (Hidden Order: HO) at about 12 K
($T_0$) above the antiferromagnetic transition temperature ($T_{\rm N}=9.4$ K).
The electronic structure change across $T_0$ was investigated with
high-precision optical reflection spectroscopy. The optical spectrum changed
from a typical metallic behavior above $T_0$ to a gap-like structure at around
15 meV below $T_0$. The gap-like structure was unchanged across $T_{\rm N}$
except for the narrowing of the Drude component of carriers due to the
suppression of magnetic fluctuations. This result suggests a slight change from
the typical metallic electronic structure above $T_0$ to that with an energy
gap near the Fermi level in the HO phase. The change in electronic structure in
the HO phase was concluded to be due to electron/valence instability.
