We investigated the electronic structures of the two-dimensional layered perovskite Sr$_{2}$textit{M}O$_{4}$ (textit{M}=4textit{d} Ru, 4textit{d} Rh, and 5textit{d} Ir) using optical spectroscopy and polarization-dependent O 1textit{s} x-ray absorption spectroscopy. While the ground states of the series of compounds are rather different, their optical conductivity spectra $sigma(omega)$ exhibit similar interband transitions, indicative of the common electronic structures of the 4textit{d} and 5textit{d} layered oxides. The energy splittings between the two $e_{g}$ orbitals, $i.e.$, $d_{3z^{2}-r^{2}}$ and $d_{x^{2}-y^{2}}$, are about 2 eV, which is much larger than those in the pseudocubic and 3textit{d} layered perovskite oxides. The electronic properties of the Sr$_{2}$textit{M}O$_{4}$ compounds are discussed in terms of the crystal structure and the extended character of the 4textit{d} and 5textit{d} orbitals.
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