We show that the QCD vacuum (without dynamical quarks) is a dual superconductor at least in the low-energy region in the sense that monopole condensation does really occur. In fact, we derive the dual Ginzburg-Landau theory (i.e., dual Abelian Higgs model) directly from the SU(2) Yang-Mills theory by adopting the maximal Abelian gauge. The dual superconductor can be on the border between type II and type I, excluding the London limit. The masses of the dual Abelian gauge field is expressed by the Yang-Mills gauge coupling constant and the mass of the off-diagonal gluon of the original Yang-Mills theory. Moreover, we can rewrite the Yang-Mills theory into an theory written in terms of the Abelian magnetic monopole alone at least in the low-energy region. Magnetic monopole condensation originates in the non-zero mass of off-diagonal gluons. Finally, we derive the confining string theory describing the low-energy Gluodynamics. Then the area law of the large Wilson loop is an immediate consequence of these constructions. Three low-energy effective theories give the same string tension.