  Optical conductivity [$\sigma(\omega)$] of the Kondo semiconductor YbB$_{12}$
has been measured over wide ranges of temperature ($T$=8$-$690 K) and photon
energy ($\hbar \omega \geq$ 1.3 meV). The $\sigma(\omega)$ data reveal the
entire crossover of YbB$_{12}$ from a metallic electronic structure at high $T$
into a semiconducting one at low $T$. Associated with the gap development in
$\sigma(\omega)$, a clear onset is newly found at $\hbar\omega$=15 meV for $T
\leq$ 20 K. The onset energy is identified as the gap width of YbB$_{12}$
appearing in $\sigma(\omega)$. This gap in \sigma(\omega)$ is interpreted as
the indirect gap, which has been predicted in the band model of Kondo
semiconductor. On the other hand, the strong mid-infrared (mIR) peak observed
in $\sigma(\omega)$ is interpreted as arising from the direct gap. The
absorption coefficient around the onset and the mIR peak indeed show
characteristic energy dependences expected for indirect and direct optical
transitions in conventional semiconductors.
