  We suggest a simple grand unified theory where the fifth dimensional
coordinate is compactified on an $S^1/(Z_2 \times Z_2')$ orbifold. This model
contains additional ${\bf 10 + \overline{10}}$, (${\bf 15 + \overline{15}}$)
and two ${\bf 24}$ Higgs multiplets in the non-supersymmetric SU(5) grand
unified theory. Although this model is non-supersymmetric theory, the gauge
coupling unification is realized due to the mass splittings of Higgs multiplets
by $S^1/(Z_2 \times Z_2')$ orbifolding. The constraints from the $b \to s
\gamma$, $\mu \to e \gamma$, and electron and muon anomalous magnetic moments
are also discussed.
