  Proton decay is one of the most important predictions of the grand unified
theory (GUT). In the supersymmetric (SUSY) GUT, proton decays via the
dimension-five operators need to be suppressed. In the $SO(10)$ model where
${\bf 10}+\overline{\bf 126}$ Higgs fields couple to fermions, neutrino
oscillation parameters including the CP-violating
Pontecorvo-Maki-Nakagawa-Sakata (PMNS) phase can be related to the Yukawa
couplings to generate the dimension-five operators in the unified framework. We
show how the suppressed proton decay depends on the PMNS phase, and stress the
importance of the precise measurements of the PMNS phase as well as the
neutrino 23-mixing angle. These become especially important if the SUSY
particles are found around less than a few TeV at LHC and proton decays are
observed at Hyper-Kamiokande and DUNE experiments in the near future.
