  In order to investigate the direct relation between confinement and chiral
symmetry breaking in QCD, we investigate the Polyakov loop in terms of the
Dirac eigenmodes in both confined and deconfined phases. Using the Dirac-mode
expansion method in SU(3) lattice QCD, we analyze the contribution of low-lying
and higher Dirac-modes to the Polyakov loop, respectively.In the confined phase
below T_c, after removing low-lying Dirac-modes, the chiral condensate $<
\bar{q} q>$ is largely reduced, however, the Polyakov loop remains almost zero
and Z_3-center symmetry is unbroken. These results indicate that the system is
still in the confined phase without low-lying Dirac-modes. By higher
Dirac-modes cut, the Polyakov loop also remains almost zero below T_c. We also
analyze the Polyakov loop in the deconfined phase above T_c. We find that the
Polyakov loop and Z_3-symmetry behavior are insensitive to low-lying and higher
Dirac-modes in both confined and deconfined phases.
