To enable used, film-like or container-like or so, bulky plastic wastes supplied in a variety of forms to be almost certainly and continuously reduced in volume and subjected to treatment.

The volume-reducing apparatus for the plastic wastes consists of a hold tank for holding the object to be treated consisting of the plastic wastes, a superheated steam feeder for supplying supereheated steam to the treated object held in the inside of the hold tank, and a screw feeder for taking out outward of the hold tank the treated object that has molten by contact with the superheated steam supplied from the superheated steam feeder. The superheated steam feeder consists of a first superheated steam feeder that supplies superheated steam from under the screw feeder and a second one that does so from above the screw feeder.