  We report our recent study of spin-orbit force between two nucleons in the
parity-odd sector from lattice QCD. To examine the quark mass dependence of
spin-orbit force, we construct it from Nambu-Bethe-Salpeter wave functions in
$^3P_0$, $^3P_1$ and $^3P_2-^3F_2$ channels calculated from lattice QCD in the
quark mass region $m_\pi$=702-411 MeV. The calculation is performed on Blue
Gene/Q at KEK by using $N_f=2+1$ PACS-CS gauge configuration generated by O(a)
improved Wilson quark action with RG improved (Iwasaki) gauge action. We find
that the potentials tend to become stronger as the quark mass decreases.
