  The pseudo-critical temperature of the confinement-deconfinement transition
and the phase transition surface are investigated by using the complex chemical
potential. We can interpret the imaginary chemical potential as the
Aharonov-Bohm phase, then the analogy of the topological order suggests that
the Roberge-Weiss endpoint would define the pseudo-critical temperature. The
behavior of the Roberge-Weiss endpoint at small real quark chemical potential
is investigated with the perturbative expansion. The expected QCD phase diagram
at complex chemical potential is presented.
