Structure of Critical Lines in Quenched Lattice QCD with the Wilson Quark Action

The structure of critical lines of vanishing pion mass for the Wilson quark action is examined in quenched lattice QCD. The numerical evidence is presented that critical lines spread into five branches beyond beta=5.6-5.7 at zero temperature. It is also shown that critical lines disappear in the deconfined phase for the case of finite temperatures.