Suppression of Proton Decay in the Missing-Partner Model for Supersymmetric SU(5) GUT

We construct a missing-partner model for supersymmetric SU(5) GUT assuming the Peccei-Quinn symmetry, in which the SU(5) gauge coupling constant remains in the perturbative regime below the gravitational scale $\sim2.4\times 10^{18}\GEV$. The Peccei-Quinn symmetry suppresses the dangerous dimension-five operators for the nucleon decay much below the limit from the present proton-decay experiments. We also stress that due to this suppression mechanism our model can accommodate even the large $\tanβ_H$ ($\sim$ 60) scenario which has been recently suggested to explain the observed value of the $m_b/m_τ$ ratio.