  Nonperturbative QCD is studied with the dual Ginzburg-Landau theory, where
color confinement is realized through the dual Higgs mechanism by QCD-monopole
condensation. We obtain a general analytic formula for the string tension. A
compact formula is derived for the screened inter-quark potential in the
presence of light dynamical quarks. The QCD phase transition at finite
temperature is studied using the effective potential formalism. The string
tension and the QCD-monopole mass are largely reduced near the critical
temperature, $T_c$. The surface tension is estimated from the effective
potential at $T_c$. We propose also a new scenario of the quark-gluon-plasma
creation through the color-electric flux-tube annihilation. Finally, we discuss
a close relation between instantons and QCD-monopoles.
