In the depolymerization and the polymerization processes using a supercritical fluid, the supercritical fluid is used as a reaction medium; therefore, the reaction medium can easily be discharged outside the system only by returning the pressure in the system to normal pressure after the reaction. Consequently, a product is easily separated from the system. The discharged reaction medium can be recovered and reused. In particular, in the case that carbon dioxide is used as the supercritical fluid, it is not feared that environment is polluted even if the solvent leaks outside the system.