A method of plastic decomposition by which a plastic can be decomposed so that the decomposition products can be utilized as a material for producing the same plastic again. The method comprises treating a thermoset resin comprising a polyester and crosslinking parts thereof with subcritical water having a temperature lower than the heat composition temperature of the thermoset resin. The method enables the thermoset resin to be decomposed into a polyhydric alcohol, a polybasic acid, and a compound having an acid residue derived from the polyester and a residue derived from the crosslinking parts, without randomly decomposing the thermoset resin. |