  We performed resonant x-ray diffraction experiments at the $L$ absorption
edges for the post-perovskite-type compound CaIrO$_{3}$ with $(t_{2g})^5$
electronic configuration. By observing the magnetic signals, we could clearly
see that the magnetic structure was a striped order with an antiferromagnetic
moment along the c-axis and that the wavefunction of a $t_{2g}$ hole is
strongly spin-orbit entangled, the $J_{\rm eff} =1/2$ state. The observed spin
arrangement is consistent with theoretical work predicting a unique
superexchange interaction in the $J_{\rm eff} =1/2$ state and points to the
universal importance of the spin-orbit coupling in Ir oxides, irrespective of
the local coordination and lattice topology. We also propose that the
non-magnetic resonant scattering is a powerful tool for unraveling an orbital
state even in a metallic iridate.
