  We study spin $\frac12$ hadronic states in quenched lattice QCD to search for
a possible $S=+1$ pentaquark resonance. Our work is the first systematic
lattice QCD study which properly carries out the following analyses: {\it (1)
the careful extraction of the first two low energy states with very high
statistics and the variational method} and {\it (2) the study of volume
dependences of eigenenergies and spectral weights to distinguish resonance
states from scattering states}.
  Simulations are carried out on $8^3\times 24$, $10^3\times 24$, $12^3\times
24$ and $16^3\times 24$ lattices at $\beta$=5.7 with the standard plaquette
gauge action and the Wilson quark action. Our result indicates the existence of
a resonance state lying slightly above the NK threshold in
$(I,J^P)=(0,\frac12^-)$ channel in quenched QCD.
