  Electroweak symmetry breaking is explored in a two Higgs doublet model based
on a six dimensional $SU(4)$ gauge-Higgs unification compactified on an
orbifold $T^2/Z_2$. The remarkable property of this model is a prediction of
realistic weak mixing angle $\sin^2 \theta_W = 1/4$ at the compactification
scale. We calculate one-loop effective potential of the Standard Model Higgs
boson from the contributions of the gauge boson and the fermion in a four-rank
totally symmetric tensor where top quark is included. We find that the
electroweak symmetry breaking certainly takes place.
