  In gauge-Higgs unification the 4D Higgs boson appears as a part of the fifth
dimensional component of gauge potentials, namely as a fluctuation mode of the
Aharonov-Bohm phase in the extra dimension. The $SO(5) \times U(1) \times
SU(3)$ gauge-Higgs unification gives nearly the same phenomenology as the
standard model (SM) at low energies. It predicts KK excited states of photon,
$Z $ boson, and $Z_R$ boson ($Z'$ bosons) around 7 - 8 TeV. Quarks and leptons
couple to these $Z'$ bosons with large parity violation, which leads to
distinct interference effects in $e^+ e^- \rightarrow \mu^+ \mu^-, q \, \bar q$
processes. At 250 GeV ILC with polarized electron beams, deviation from SM can
be seen at the 3 - 5 sigma level even with 250 fb$^{-1}$ data, namely in the
early stage of ILC. Signals become stronger at higher energies. Precision
measurements of interference effects at electron-positron colliders at energies
above 250 GeV become very important to explore physics beyond the standard
model.
