  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 while $DD$ interaction is repulsive, and there is no
bound $\frac 12^+$ pentaquark state. A bound pentaquark $\Theta^+$ can only be
obtained with unphysically strong vector mesonic coupling constants.
