  The cosmological QCD phase transition is studied in terms of the color
confinement at finite temperature using the dual Higgs theory of QCD. The
confinement force is largely reduced at high temperature, which leads to the
swelling of hadrons. We derive analytical formulae for the surface tension and
the boundary thickness of the mixed phase from the effective potential at
$T_c$. We predict a large reduction of the glueball mass near $T_c$. We
investigate also the process of the hadron-bubble formation in the early
Universe.
