To reliably crush even large size and solid plastics such as marine floats without stagnating the plastics.

The volume-decreasing apparatus for plastics is provided with a hopper 3 for chuting plastics and a treatment chamber 2 for crushing and pressurizing plastics fed from the hopper 3 and decreasing the volume of the plastics and the treatment chamber 2 is provided with a rotary shaft 8 in which a crushing arm 6 for crushing the plastics and screw blades 7 for sending the plastics forward are installed, crushing plates 9 arranged like vertical stripes at a prescribed gap in the back and forth direction so as to avoid interference between the crushing arm 6 and themselves, and a tapered pressurizing cylinder 10 surrounding the screw blades 7 in the front end part of the rotary shaft 8. In such an apparatus, a stirring apparatus 30 for stirring not-yet-crushed plastics is installed in the feeding portion where plastics are sent from the hopper 3 to the treatment chamber 2.