To achieve excellent thermal efficiency, by disposing a stopper for a lid comprising a plurality of rods arranged at a predetermined distance from a heater.

A predetermined amount of waste plastics 1D, such as waste foamed polystyrene, is charged into a volume reduction room 1A and, a fall rid 1E of a predetermined weight having a structure formed by cladding of two sheets of steel on both surfaces of a spacer is set on waste plastics 1D. The plastics 1D is brought into contact and pressed to a heater 1D by the weight of the fall lid itself so that the plastics 1D is reduced in volume by heating at a temperature of not higher than 240°C. The fall lid 1F is stopped at a position somewhat above two or more of rod stoppers 2H disposed at a predetermined distance from heater 1F. And then the temperature of heater 1E is increased to a temperature not more than 600°C, especially to about 550°C, and at the same time, the leakage of gases evolved from the plastics is prevented by flashing an inert gas such as N2 gas at the speed of about 10-500ml/min so as to seal the clearance between the fall lid 1E and the wall of a body, so that a decomposition liquid containing no tar substances is obtained in high yield.