  We investigate the high-energy behavior of the scattering amplitudes in the
extra dimensional gauge theory where the gauge symmetry is broken by the
boundary condition. We study, in particular, the 5-dimensional SU(5) grand
unified theory whose 5th-dimensional coordinate is compactified on S^1/Z_2. We
pay attention to the gauge symmetry compatible with the boundary condition on
the orbifold and give the BRST formalism of the 4D theory which is obtained
through integration of the 5D theory along the extra dimension. We derive the
4D equivalence theorem on the basis of the Slavnov-Taylor identities. We
calculate the amplitudes of the process including four massive gauge bosons in
the external lines and compare them with the ones for the connected reactions
where the gauge fields are replaced by the corresponding would-be NG-like
fields. We explicitly confirm the equivalence theorem to hold.
