  Dimension-five operators for nucleon decays are suppressed in the modified
missing doublet (MMD) model in the supersymmetric SU(5) grand unification. We
show that nonrenormalizable interactions decrease the unification scale in the
MMD model which increases the nucleon decay rate of dimension-six operators by
a significant amount. We find that the theoretical lower bound on the proton
life time $\tau(p \to \e^+ \pi^0)$ is within the observable range at
SuperKamiokande.
