  The mechanism of color confinement as a consequence of an unbroken
non-abelian gauge symmetry and asymptotic freedom is elucidated and compared
with that of other models based on an analogy with the type II superconductor.
It is demonstrated that a sufficient condition for color confinement is given
by $Z_3^{-1}=0$ where $Z_3$ denotes the renormalization constant of the color
gauge field. It is shown that this condition is actually satisfied in quantum
chromodynamics and that some of the characteristic features of other models
follow from it.
