  We analyze the electroweak phase transition at finite temperature in a model
of gauge-Higgs unification where the fermion mass hierarchy including top quark
mass, a viable electroweak symmetry breaking and an observed Higgs mass are
successfully reproduced. To study the phase transition, we derive the general
formula of the 1-loop effective potential at finite temperature by using the
$\zeta$ function regularization method. It is remarkable that the functions
determining the Kaluza-Klein mass spectrum have only to be necessary in
calculations. This potential can be applicable to any higher dimensional theory
in flat space where one extra spatial dimension is compactified. Applying to
our model of gauge-Higgs unification, the strong first phase transition
compatible with 125 GeV Higgs mass is found to happen.
