To provide a method for manufacturing a useful substance from a plastic as a raw material which affords coke of stable high qualities.

The method for manufacturing a useful substance from a plastic as a raw material comprises a step for mixing fuel oil B with a plastic A, and heating and melting the resultant mixture, and a step for hydrocracking the heat-melted mixture in a reaction column 2. For manufacturing coke, the method for manufacturing a useful substance from a plastic as a raw material further comprising a step for separating in a separation tank 3 a gas component from the product obtained by hydrocracking, a step for separating in a distillation column 4 a product oil having a boiling point of at least 280°C from the product after separating the gas component and a step for coking the product oil in coking drums 7a and 7b thereby to obtain coke is preferably employed.

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