To provide a method for producing a plastic decomposed oil whereby the plastic decomposed oil can be obtained even when temperature is lower than the conventional, and a reaction vessel used has a lower pressure resistance.

The method for producing a plastic decomposed oil comprises allowing waste plastics, carbon dioxide, a hydrogen gas and a hydrogenation catalyst to reside in the same reaction vessel and decomposing the plastics under conditions of a temperature of at least 150°C and a pressure of at least 5 MPa. It is desirable that the carbon dioxide is used in an amount of at least 10 pts.wt. per 100 pts.wt. waste plastics.