  Resonant x-ray diffraction experiments were performed for the metallic
iridium oxide IrO$_{2}$. We observed anisotropic tensor of susceptibility (ATS)
scattering, the spectrum of which shows a sharp contrast between the $L_{3}$
and $L_{2}$ edges. At the $L_{3}$ edge, resonance excitations were clearly
observed from the core 2$p$ orbitals to both the 5$d$ $t_{2g}$ and $e_{g}$
orbitals. In contrast, the resonance mode associated with 5$d$ $t_{2g}$
orbitals was indiscernible at the $L_{2}$ edge. This contrasting behavior
indicates that Ir 5$d$ $t_{2g}$ orbitals are fairly close to the $J_{\rm eff}$
= 1/2 state due to the strong spin--orbit coupling in 5$d$ transition metal
ions, as in the Mott insulator Sr$_{2}$IrO$_{4}$. Our results clearly
demonstrate that ATS scattering is a useful probe for investigating complex
orbital states in a metallic state. Such states induce novel phenomena such as
the spin Hall effect.
