  We investigate the relevant gluon-momentum region for confinement in lattice
QCD on $16^4$ at $\beta$=5.7, 5.8 and 6.0, based on the Fourier expansion. We
find that the string tension $\sigma$, i.e., the confining force, is almost
unchanged even after removing the high-momentum gluon component above 1.5GeV in
the Landau gauge. In fact, the confinement property originates from the
low-momentum gluon component below 1.5GeV, which is the upper limit to
contribute to $\sigma$. In the relevant region, smaller gluon momentum
component is more important for confinement. Next, we develop a manifestly
gauge-covariant expansion of the QCD operator such as the Wilson loop, using
the eigen-mode of the QCD Dirac operator $\gamma^\mu D^\mu$. With this method,
we perform a direct analysis of the correlation between confinement and chiral
symmetry breaking in lattice QCD on $6^4$ at $\beta$=5.6. As a remarkable fact,
the confinement force is almost unchanged even after removing the low-lying
Dirac modes, which are responsible to chiral symmetry breaking. This indicates
that one-to-one correspondence does not hold for between confinement and chiral
symmetry breaking in QCD.
