Pressure-induced continuous phase transition of charge-density-wave state in a linear-chain complex

[Pt(en${)}_2$][Pt(en${)}_2$ ${\mathrm{Cl}}_2$](${\mathrm{ClO}}_4$ ${)}_4$ (en=ethylenediamine) is found to undergo a novel phase transition from the charge-density-wave (CDW) state to a new phase under hydrostatic pressure. The new phase appears at 3 GPa and coexists with the CDW state up to 6 GPa, at least. The volume ratio between the two coexisting phases measured by Raman scattering spectroscopy changes continuously with pressure. The result is discussed in terms of the theoretical approach to the polymorphic phase transition developed recently by Bassler, Sasaki, and Griffiths. It is suggested that kink solitons play an important role in this phase transition.