PURPOSE: To recover oil of good quality utilizing wastes, e.g. agricultural films, containers and daily miscellaneous goods, by decomposing polyethylene based synthetic resin under heating at a given temperature.

CONSTITUTION: Polyethylene based synthetic resin wastes are pulverized or converted into semimolten or highly viscous materials, which are then fed from a hopper 13 in a constant amount into a decomposition vessel 6. A heat medium 4 is heated within 450W600°C temperature range by a heating mechanism 5, and the materials are stirred with a stirring mechanism 11 while keeping the interior of the vessel 6 at a constant temperature within 400W500°C temperature range to melt the polyethylene based synthetic resin in the vessel 6 into a liquid, which is then decomposed thermally into low-molecular materials. Thus, the resultant vapor-like products are passed through a discharging path 9 through a secondary decomposition column 17 and recovered as an oil.