  We study the flux-tube solution in the dual Ginzburg-Landau (DGL) theory and
apply it to the understanding of quark-gluon-plasma (QGP) formation in the
ultra-relativistic heavy ion collision as the multi-flux-tube system. It is
concluded that the annihilation process of two flux-tubes can contribute to the
QGP formation with the energy deposition $\sim 4 GeV/fm^3$. Furthermore, we
study the glueball as the flux-tube ring by combining the DGL theory with the
string theory, and find the mass and the size as $\sim 1.2 GeV$ and $\sim 0.22
fm$, respectively.
