  The Higgs boson mass and top quark mass imply that the Higgs quartic coupling
vanishes around the scale of $10^9 - 10^{13}$ GeV, depending on the precise
value of the top quark mass. The vanishing quartic coupling can be naturally
addressed if the Higgs field originates from a 5-dimensional gauge field and
the 5th dimension is compactified at the scale of the vanishing Higgs quartic
coupling, which is a scenario based on gauge-Higgs unification. We present a
general prediction of the scenario on the proton decay process $p \to \pi^0
e^+$. In many gauge-Higgs unification models, the 1st generation fermions are
localized towards an orbifold fixed point in order to realize the realistic
Yukawa couplings. Hence, four-fermion operators responsible for the proton
decay can appear with a suppression of the 5-dimensional Planck scale (not the
4-dimensional Planck scale). Since the 5-dimensional Planck scale is connected
to the compactification scale, we have a correlation between the proton partial
decay width and the top quark mass. We show that the future Hyper-Kamiokande
experiment may discover the proton decay if the top quark pole mass is larger
than about 172.5 GeV.
