  We construct a supersymmetric flipped SU(5) grand unified model that
possesses an $R$ symmetry. This $R$ symmetry forbids dangerous
non-renormalizable operators suppressed by a cut-off scale up to sufficiently
large mass dimensions so that the SU(5)-breaking Higgs field develops a vacuum
expectation value of the order of the unification scale along the $F$- and
$D$-flat directions, with the help of the supersymmetry-breaking effect. The
mass terms of the Higgs fields are also forbidden by the $R$ symmetry, with
which the doublet-triplet splitting problem is solved with the missing partner
mechanism. The masses of right-handed neutrinos are generated by
non-renormalizable operators, which then yield a light neutrino mass spectrum
and mixing through the seesaw mechanism that are consistent with neutrino
oscillation data. This model predicts one of the color-triplet Higgs multiplets
to lie at an intermediate scale, and its mass is found to be constrained by
proton decay experiments to be $\gtrsim 5 \times 10^{11}$ GeV. If it is
$\lesssim 10^{12}$ GeV, future proton decay experiments at Hyper-Kamiokande can
test our model in the $p \to \pi^0 \mu^+$ and $p \to K^0 \mu^+$ decay modes, in
contrast to ordinary grand unified models where $p \to \pi^0 e^+ $ or $p \to
K^+ \bar{\nu}$ is the dominant decay mode. This characteristic prediction for
the proton decay branches enables us to distinguish our model from other
scenarios.
