  We study the QCD phase transition at finite temperature and discuss the
hadron bubble formation in the dual Ginzburg-Landau theory, which is an
effective theory of QCD and describes the color confinement via QCD-monopole
condensation. We formulate the effective potential at various temperatures and
find that thermal effects reduce the QCD-monopole condensate and bring a
first-order deconfinement phase transition. Based on this effective potential
at finite temperature, we investigate properties of hadron bubbles created in
the early Universe and discuss the hadron bubble formation process.
