  A novel Lattice QCD (LQCD) method to determine the quark-diquark ($q$-$D$)
interaction potential together with the diquark mass ($m_D$) is proposed.
Similar to the HAL QCD method, $q$-$D$ potential is determined by demanding it
to reproduce the $q$-$D$ equal-time Nambu-Bethe-Salpeter (NBS) wave function.
To do this, it is necessary to use the masses of the quark and the diquark as
inputs, which however are not straightforwardly obtained because of the color
confinement of QCD. In this work, masses of quark and diquark are determined by
demanding that the p-wave spectrums from the two-point correlators be
reproduced by the potentials for $c$-$\bar{c}$ and $q$-$D$ sectors determined
from the NBS wave functions. Numerical calculations are performed by using 2+1
flavor QCD gauge configurations with the pion mass $m_\pi\simeq 700$ MeV
generated by PACS-CS collaboration. We apply our method to the $c$-$\bar c$
system and the charm-diquark system ($\Lambda_c$ baryon) to obtain the charm
quark mass, diquark mass and the $c$-$D$ potential. Our preliminary analysis
leads to the diquark mass $m_D \simeq 1.127$ GeV which is roughly consistent
with a naive estimate based on the constituent quark picture, i.e., $m_{D}
\simeq m_{\rho} \simeq 1.12$ GeV and $m_{D} \simeq 2m_N/3 \simeq 1.06$ GeV.
