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We generate a random instanton vacuum with various densities and size distributions.We perform numerically the maximally abelian gauge fixing of these configurations in order to find monopole trajectories induced by instantons. We find that instanton-induced monopole loops form enormous clusters occupying the whole physical volume, provided instantons are sufficiently dense. It indicates that confinement might be caused by instantons.

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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 ()ϳ Ϫ5 for large size . We first investigate a monopole clustering appearing in the maximally Abelian ͑MA͒ 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 antiquark in the multi-instanton system using the SU͑2͒ lattice with a total volume of Vϭ(10 fm) 4 and a lattice spacing of aϭ0.05 fm. We extract the string tension from the 5ϫ10 6 measurements of Wilson loops. With an instanton density of (N/V)ϭ(1/fm) 4 and an average instanton size of ϭ0.4 fm, the multi-instanton system provides a string tension of about 0.4 GeV/fm.

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