  We investigated the electronic structures of the perovskite-type 4$d$
transition metal oxides Sr$M$O$_3$ ($M$ = Zr, Mo, Ru, and Rh) using their
optical conductivity spectra $\sigma (\omega)$. The interband transitions in
$\sigma (\omega)$ are assigned, and some important physical parameters, such as
on-site Coulomb repulsion energy $U$, charge transfer energy $\Delta_{pd}$, and
crystal field splitting $10Dq$, are estimated. It is observed that $\Delta
_{pd}$ and 10$Dq$ decrease systematically with the increase in the atomic
number of the 4$d$ transition metal. Compared to the case of 3$d$ transition
metal oxides, the magnitudes of $\Delta_{pd}$ and 10$Dq$ are larger, but those
of $U$ are smaller. These behaviors can be explained by the more extended
nature of the orbitals in the 4$d$ transition metal oxides.
