Structure of critical lines in quenched lattice QCD with the Wilson quark action

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