The pressure dependence of the Raman and luminescence spectra of $2H\ensuremath{-}{\mathrm{PbI}}_{2}$ has been studied under hydrostatic pressure up to 1.5 GPa at 77 K by using a diamond-anvil cell. A structural phase transition occurs at a pressure of ${P}_{\mathrm{PT}}=0.56\ifmmode\pm\else\textpm\fi{}0.01 \mathrm{GPa}.$ From the analysis of the Raman spectra, it is proposed that the unit cell of the high-pressure phase is a $2\ifmmode\times\else\texttimes\fi{}2$ supercell of that of the low-pressure phase. The peak energies of the free exciton recombination and donor-acceptor pair bands shift continuously across the phase transition, indicating that relative positions of atoms and the nature of interatomic bonding hardly change at the phase transition. The pressure dependence of the fundamental absorption edge which is well described by the Urbach rule has also been examined at 77 and 300 K. At ${P}_{\mathrm{PT}}$ the steepness constant ${\ensuremath{\sigma}}_{0}$ in the Urbach tail changes drastically from 1.70 to 0.63. This result is consistent with the proposed doubling of unit-cell dimensions at ${P}_{\mathrm{PT}}.$