  We study color confinement properties of the multi-instanton system, which
seems to carry an essence of the nonperturbative QCD vacuum. Here we assume
that the multi-instanton system is characterized by the infrared suppression of
instantons as $f(\rho)\sim \rho^{-5}$ for large size $\rho$. We first
investigate a monopole-clustering appearing in the maximally abelian (MA) gauge
by considering the correspondence between instantons and monopoles. In order to
clarify the infrared monopole properties, we make the ``block-spin''
transformation for monopole currents. The feature of monopole trajectories
changes drastically with the instanton density. At a high instanton density,
there appears one very long and highly complicated monopole loop covering the
entire physical vacuum. Such a global network of long-monopole loops resembles
the lattice QCD result in the MA gauge. Second, we observe that the SU(2)
Wilson loop obeys an area law and the static quark potential is approximately
proportional to the distance $R$ between quark and anti-quark in the
multi-instanton system using the SU(2) lattice with a total volume of $V=(10
fm)^4$ and a lattice spacing of $a=0.05 fm$. We extract the string tension from
the $5 \times 10^{6}$ measurements of Wilson loops. With an instanton density
of $(N/V)=(1/fm)^4$ and a average instanton size of $\bar{\rho}=0.4 fm$, the
multi-instanton system provides the string tension of about $0.4 GeV/fm$.
