  We give an outline of a recent proof that the low-energy effective gauge
theory exhibiting quark confinement due to magnetic monopole condensation can
be derived from QCD without any specific assumption. We emphasize that the
low-energy effective abelian gauge theories obtained here give the dual
description of the same physics in the low-energy region. They show that the
QCD vacuum is nothing but the dual (type II) superconductor.
