  Penta-quark (5Q) baryons are studied using anisotropic lattice QCD for
high-precision measurement of temporal correlators. A non-NK-type interpolating
field is employed to study the 5Q states with J^P=1/2^{\pm} and I=0. In
J^P=1/2^+ channel, the lowest-lying state is found at m_{5Q} \simeq 2.25 GeV,
which is too massive to be identified as the Theta^+(1540). In J^P=1/2^-
channel, the lowest-lying state is found at m_{5Q} \simeq 1.75 GeV. To
distinguish a compact 5Q resonance state from an NK scattering state, a new
method with ``hybrid boundary condition (HBC)'' is proposed. As a result of the
HBC analysis, the observed state in the negative-parity channel turns out to be
an $NK$ scattering state.
