  Using the SU(2) lattice QCD, we formulate the dual Wilson loop and study the
dual Higgs mechanism induced by monopole condensation in the maximally abelian
(MA) gauge, where QCD is reduced into an abelian gauge theory including the
electric current $j_\mu$ and the monopole current $k_\mu$. After the abelian
projection in the MA gauge, the system can be separated into the photon part
and the monopole part corresponding to the separation of $j_\mu$ and $k_\mu$,
respectively. We study here the monopole part (the monopole-current system),
which is responsible to the electric confinement. Owing to the absence of
electric currents, the monopole part is naturally described using the dual
gluon field $B_\mu$ without the Dirac-string singularity. Defining the dual
Wilson loop from the dual gluon $B_\mu$, we find the perimeter law of the dual
Wilson loop in the lattice QCD simulation. In the monopole part in the MA
gauge, the inter-monopole potential is found to be flat, and can be fitted as
the Yukawa potential in the infrared region after the subtraction of the
artificial finite-size effect on the dual Wilson loop. From more detailed
analysis of the inter-monopole potential considering the monopole size, we
estimate the effective dual-gluon mass $m_B \simeq 0.5$GeV and the effective
monopole size $R \simeq 0.2$fm. The effective mass of the dual gluon field at
the long distance can be regarded as an evidence of ``infrared monopole
condensation''.
