Abstract: The present invention provides a method for decomposing a plastic so as to recycle useful substances therefrom as raw materials for a similar plastic. In particular, the present invention provides a method for decomposing a plastic, comprising the step of treating a thermosetting resin comprising a polyester and its crosslinking moiety, with subcritical water of a temperature lower than the thermal decomposition temperature of the thermosetting resin. According to the present invention, the thermosetting resin can be decomposed into a polyhydric alcohol, a polybasic acid and a compound comprising an acid residue derived from the polyester and a residue derived from the crosslinking moiety, without causing a random decomposition of the thermosetting resin.