  A Bethe-Salpeter-Faddeev (BSF) calculation is performed for the pentaquark
$\Theta^+$ in the diquark picture of Jaffe and Wilczek in which $\Theta^+$ is a
diquark-diquark-${\bar s}$ three-body system. Nambu-Jona-Lasinio (NJL) model is
used to calculate the lowest order diagrams in the two-body scatterings of
${\bar s}D$ and $D D$. With the use of coupling constants determined from the
meson sector, we find that ${\bar s}D$ interaction is attractive in s-wave
while $DD$ interaction is repulsive in p-wave. With only the lowest three-body
channel considered, we do not find a bound $ \frac 12^+$ pentaquark state.
Instead, a bound pentaquark $\Theta^+$ with $ \frac 12^-$ is obtained with a
unphysically strong vector mesonic coupling constants.
