  A simple higher dimensional mechanism of the doublet-triplet splitting is
presented in a five dimensional supersymmetric SU(5) GUT on $S^1/Z_2$. The
splitting of multiplets is realized by a mass term of Higgs hypermultiplet
which explicitly breaks SU(5) gauge symmetry. Depending on the sign of mass,
zero mode Higgs doublets and triplets are localized on the either side of the
fixed points. The mass splitting is realized due to the difference of
magnitudes of the overlap with a brane localized or a bulk singlet field. No
unnatural fine-tuning of parameters is needed. The proton stability is ensured
by locality {\em without symmetries}. As well as a conventional mass splitting
solution, it is shown that the weak scale Higgs triplet is consistent with the
proton stability. This result might provide an alternative signature of GUT in
future collider experiments.
