To provide a method and a small-sized unit therefor simple in scheme for converting plastics to oil, capable of removing both hydrogen cyanide and hydrogen halide generated in association with the pyrolysis of plastic wastes.

This method comprises the following process: plastic wastes are pyrolyzed in a pyrolysis tank, the pyrolysis gas generated thereby is cooled and converted to oil, which is then collected, the pyrolysis gas that has not been converted to oil is incinerated; wherein prior to incinerating the gas, it is passed through a halogen-immobilizing material to immobilize and remove the hydrogen halide in the gas. The other objective unit comprises a cooling tower 8 for converting the pyrolysis gas generated in the pyrolysis tank to oil, a means 9 for incinerating the pyrolysis gas that has not been converted to oil in the cooling tower 8, a dehalogenating column 7 packed with the halogen-immobilizing material 6 in a path 15 for introducing the gas into the means 9, and a gas blowing-off preventive means 12 provided on a path through which the product oil is introduced into a relevant tank.