  We study the proton lifetime in the $SO(10)$ Grand Unified Theory (GUT),
which has the left-right (LR) symmetric gauge theory below the GUT scale. In
particular, we focus on the minimal model without the bi-doublet Higgs field in
the LR symmetric model, which predicts the LR-breaking scale at around
$10^{10\text{--}12}$ GeV. The Wilson coefficients of the proton decay operators
turn out to be considerably larger than those in the minimal $SU(5)$ GUT model
especially when the Standard Model Yukawa interactions are generated by
integrating out extra vector-like multiplets. As a result, we find that the
proton lifetime can be within the reach of the Hyper-Kamiokande experiment even
when the GUT gauge boson mass is in the $10^{16\text{--}17}$ GeV range. We also
show that the mass of the extra vector-like multiplets can be generated by the
Peccei-Quinn symmetry breaking in a consistent way with the axion dark matter
scenario.
