To divide the molecules of a high-molecular waste mineral oil or a waste synthetic resin in an excited state to convert the oil or resin into a low-molecular product by heating the oil or resin to a high temp. while simultaneously bringing it into contact with high-temp. steam.

The figure illustrates a thermal decomposition vessel for this purpose. A waste oil mixture is supplied from a supply port 16 into a noncatalytic thermal decomposition vessel 1 with a pump 30 as that the liq. level in the vessel is kept const. The vessel 1 is heated by hot air of about 800°C sent from a hot air generator 5, and simultaneously steam of about 400°C is blown from a nozzle 11 into the oil mixture. Thus, a high-mol.-wt. waste mineral oil or a waste synthetic resin changes into a low-boiling substance and vaporises. Here, 'high mol.wt.' means a mol.wt. of 600-1,200, and 'low mol.wt.' means a mol.wt. of about 300. The vapor generated is sent to a distillation column 19 to be separated into low-molecular and medium- molecular oils. Steam and the low-molecular oil are taken out as vapors from the top of the column and sent to a condenser 20 to be liquefied. Then, the resultant liq. is separated with an oil-water separator 21 and sent to an oil tank 22 and a water tank 23.