  Magnetic properties of single crystalline SmRu$_{2}$Al$_{10}$ have been
investigated by electrical resistivity, magnetic susceptibility, and specific
heat. We have confirmed the successive magnetic phase transitions at
$T_{\text{N}}=12.3$ K and $T_{\text{M}}=5.6$ K. Resonant x-ray diffraction has
also been performed to study the magnetic structures. Below $T_{\text{N}}$, the
Sm$^{3+}$ moments order in an incommensurate structure with $q_1=(0, 0.759,
0)$. The magnetic moments are oriented along the orthorhombic $b$ axis, which
coincides with the magnetization easy axis in the paramagnetic phase. A very
weak third harmonic peak is also observed at $q_3=(0, 0.278, 0)$. The
transition at $T_{\text{M}}$ is a lock-in transition to the commensurate
structure described by $q_1=(0, 0.75, 0)$. A well developed third harmonic peak
is observed at $q_3=(0, 0.25, 0)$. From the discussion of the magnetic
structure, we propose that the long-range RKKY interaction plays an important
role, in addition to the strong nearest neighbor antiferromagnetic interaction.
