  Color confinement can be understood by the dual Higgs theory, where monopole
condensation leads to the exclusion of the electric flux from the QCD vacuum.
We study the role of the monopole for color confinement by investigating the
monopole current system. When the self-energy of the monopole current is small
enough, long and complicated monopole world-lines appear, which is a signal of
monopole condensation. In the dense monopole system, the Wilson loop obeys the
area-law, and the string tension and the monopole density have similar behavior
as the function of the self-energy, which seems that monopole condensation
leads to color confinement. On the long-distance physics, the monopole current
system almost reproduces essential features of confinement properties in
lattice QCD. In the short-distance physics, however, the monopole-current
theory would become nonlocal and complicated due to the monopole size effect.
This monopole size would provide a critical scale of QCD in terms of the dual
Higgs mechanism.
