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.
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