  We study the unitarity bounds of the scattering amplitudes in the extra
dimensional gauge theory where the gauge symmetry is broken by the boundary
condition. The estimation of the amplitude of the diagram including four
massive gauge bosons in the external lines shows that the asymptotic power
behavior of the amplitude is canceled. The calculation will be done in the 5
dimensional standard model and the SU(5) grand unified theory, whose 5th
dimensional coordinate is compactified on $S^1/Z_2$. The broken gauge theories
through the orbifolding preserve the unitarity at high energies similarly to
the broken gauge theories where the gauge bosons obtain their masses through
the Higgs mechanism. We show that the contributions of the Kaluza-Klein states
play a crucial role in conserving the unitarity.
