  Infrared optical conductivity [$\sigma(\omega)$] of the intermediate valence
compound YbAl$_3$ has been measured at temperatures 8 K $\leq T \leq$ 690 K to
study its microscopic electronic structures. Despite the highly metallic
characters of YbAl$_3$, $\sigma(\omega)$ exhibits a clear pseudogap (strong
depletion of spectral weight) of about 60 meV below 40 K. It also shows a
strong mid-infrared peak centered at $\sim$ 0.25 eV. Energy-dependent effective
mass and scattering rate of the carriers obtained from the data indicate the
formation of a heavy-mass Fermi liquid state. These characteristic results are
discussed in terms of the hybridization states between the Yb 4$f$ and the
conduction electrons. It is argued, in particular, that the pseudogap and the
mid-infrared peak result from the indirect and the direct gaps, respectively,
within the hybridization state. band.
