To provide a method for producing a regeneration raw material for re-synthesizing an unsaturated polyester resin by decomposing a waste material of molded articles containing an unsaturated polyester resin, the method aiming to generate a regeneration raw material by decomposing a waste material of molded articles containing an unsaturated polyester resin by using a lower alcohol in a sub-critical state to a super-critical state and obtaining a regenerated resin in good quality by using the regeneration raw material, and to provide a method for re-synthesizing the unsaturated polyester resin, and a method for producing an unsaturated polyester resin.

The method includes a step S2 of decomposing a waste material of molded articles containing an unsaturated polyester resin by using a lower alcohol in a sub-critical state to a super-critical state without adding a catalyst, so as to generate a phthalic acid ester and glycol, which are soluble in the lower alcohol, and a compound of polystyrene and maleic acid, which is insoluble in the lower alcohol.