  In the supersymmetric SU(5) grand unified theory whose gauge symmetry is
broken by virtue of the Hosotani mechanism, the huge mass splitting between the
colored Higgs triplet and the electroweak Higgs doublet superfields is
naturally realized. As a byproduct, the existence of adjoint chiral superfields
with masses of the order of the supersymmetry breaking scale is predicted,
leading to the Higgs sector that contains an SU(2)_L triplet chiral multiplet
with hypercharge zero and a neutral singlet one in addition to the two SU(2)_L
doublets of the minimal supersymmetric standard model. We focus on the Higgs
sector and investigate to what extent the couplings of the standard model-like
Higgs boson and the masses of the additional Higgs bosons deviate from those in
the Standard Model and other models due to the new triplet and singlet chiral
multiplets. We show that this model can be distinguished using precision
measurements of couplings and masses of the Higgs sector particles and serves
as a good example of grand unification testable at colliders such as the
luminosity up-graded Large Hadron Collider and future electron-positron
colliders.
