  We investigate the mass spectrum of the pentaquark baryon ($ udud \bar s $)
in quenched lattice QCD with exact chiral symmetry. Using 3 different
interpolating operators, we measure their $ 3 \times 3 $ correlation matrix and
obtain the eigenvalues $ A^{\pm} (t) $ with $ \pm $ parity, for 100 gauge
configurations generated with Wilson gauge action at $ \beta = 6.1 $ on the $
20^3 \times 40 $ lattice. For the lowest-lying $ J^P = 1/2^- $ state, its
effective mass is almost identical to that of the KN s-wave, while for the
lowest-lying $ J^P = 1/2^+ $ state, its effective mass is smaller than that of
the KN p-wave, especially for the regime $ m_u < m_s $. By chiral extrapolation
(linear in $m_\pi^2$) to $ m_\pi = 135 $ MeV, we obatin the masses of the
lowest-lying states: $ m(1/2^-) = 1424(57) $ MeV and $ m(1/2^+) = 1562(121) $
MeV, in agreement with the masses of $ m_K + m_N \simeq 1430 $ MeV and $
\Theta^+(1540) $ respectively.
