  We study the dual Wilson loop and the inter-monopole potential(the static
potential between the color magnetic monopoles) in the maximally abelian gauge
to clarify the dual Higgs mechanism induced by monopole condensation. There is
no (color-)electric current in the monopole part, which includes the essence of
the nonperturbative QCD, and hence the system can be described by the dual
gauge field $B_\mu$ without the singularity like the Dirac string. We find that
the dual Wilson loop seems to obey the perimeter law, and the inter-monopole
potential becomes Yukawa-type in the infrared region. From the inter-monopole
potential, we estimate the dual gluon mass $m_B$ and the effective size $R$ of
the monopole: $m_B \simeq 0.5$GeV, $R \simeq 0.35$fm.
