To provide a method of preventing a solution piping for supplying a plastic solution to a pyrolysis vessel from being blocked due to the retention of impurities.

A method of stopping a pyrolysis system in which a plastic solution is supplied through a solution piping 20 to a pyrolysis vessel 2 to be subjected to the pyrolysis and the resultant pyrolysis gas is cooled to produce a produced oil comprises stopping the supply of the plastic solution to the pyrolysis vessel 2 and thereafter supplying the produced oil as a cleaning liquid to the solution piping 20 to discharge the plastic solution remaining in the inside of the solution piping 20 to the side of the pyrolysis vessel 2.