  We estimate the mass of the pentaquark state with QCD sum rules and find that
pentaquark states with isospin $I=0, 1, 2$ lie close to each other around
$(1.55\pm 0.15)$ GeV. The experimentally observed baryon resonance $\Theta^+
(1540)$ with $S=+1$ can be consistently identified as a pentaquark state if its
$J^P={1\over 2}^-$. Such a state is expected in QCD. If its parity is positive,
this pentaquark state is really exotic. Now the outstanding issue is to
determine its quantum numbers experimentally.
