  The anisotropic electronic structure responsible for the antiferromagnetic
transition in CeRu$_2$Al$_{10}$ at the unusually high temperature of $T_0$ = 28
K was studied using optical conductivity spectra, Ce 3d X-ray photoemission
spectra, and band calculation. It was found that the electronic structure in
the $ac$ plane is that of a Kondo semiconductor, whereas that along the b axis
has a nesting below 32 K (slightly higher than $T_0$). These characteristics
are the same as those of CeOs$_2$Al$_{10}$ [S. Kimura {\it et al.}, Phys. Rev.
Lett. 106, 056404 (2011).]. The $c$-$f$ hybridization intensities between the
conduction and $4f$ electrons of CeRu$_2$Al$_{10}$ and CeOs$_2$Al$_{10}$ are
weaker than that of CeFe$_2$Al$_{10}$, showing no magnetic ordering. These
results suggest that the electronic structure with one-dimensional weak $c$-$f$
hybridization along the b axis combined with two-dimensional strong
hybridization in the $ac$ plane causes charge-density wave (CDW) instability,
and the CDW state then induces magnetic ordering.
