  We investigate the Polyakov loop effects on the QCD phase diagram by using
the strong-coupling (1/g^2) expansion of the lattice QCD (SC-LQCD) with one
species of unrooted staggered quark, including O}(1/g^4) effects. We take
account of the effects of Polyakov loop fluctuations in Weiss mean-field
approximation (MFA), and compare the results with those in the Haar-measure MFA
(no fluctuation from the mean-field). The Polyakov loops strongly suppress the
chiral transition temperature in the second-order/crossover region at small
chemical potential, while they give a minor modification of the first-order
phase boundary at larger chemical potential. The Polyakov loops also account
for a drastic increase of the interaction measure near the chiral phase
transition. The chiral and Polyakov loop susceptibilities have their peaks
close to each other in the second-order/crossover region. In particular in
Weiss MFA, there is no indication of the separated deconfinement transition
boundary from the chiral phase boundary at any chemical potential. We discuss
the interplay between the chiral and deconfinement dynamics via the bare quark
mass dependence of susceptibilities.
