  We investigate the QCD phase diagram by using the strong-coupling expansion
of the lattice QCD with one species of staggered fermion and the Polyakov loop
effective action at finite temperature (T) and quark chemical potential (mu).
We derive an analytic expression of effective potential Feff including both the
chiral (U(1)) and the deconfinement (Z_Nc) dynamics with finite coupling
effects in the mean-field approximation. The Polyakov loop increasing rate
(dl/dT) is found to have two peaks as a function of T for small quark masses.
One of them is the chiral-induced peak associated with the rapid decrease of
the chiral condensate. The temperature of the other peak is almost independent
of the quark mass or chemical potential, and this peak is interpreted as the
Z_Nc-induced peak.
