To finely grind a plastic material at a low cost.

The plastic grinder is provided with a receiving vessel 10 for receiving and enclosing a plastic material therein, plural evacuation means for evacuating the inside of the receiving vessel 10, a cooling means 30 for cooling the plastic material to its brittle temperature and a grinding means 40 for grinding the plastic material cooled by the cooling means 30, wherein the receiving vessel 10 is provided with a main chamber 10A for cooling the plastic material, spare chambers 10B and 10C from or to each of which the plastic material can be transferred to or from the main chamber 10A, and three evacuation means PA, PB and PC placed in the main chamber 10A and spare chambers 10B and 10c, respectively. Thus, since the main chamber 10A and spare chambers 10B and 10C are evacuated by their respective evacuation means PA, PB and PC and the plastic material can be transferred from or to each of the spare chambers 10B and 10C to or from the main chamber 10A, each of the spare chambers 10B and 10C is interposed between the main chamber 10A and the outside air, and accordingly, the evacuated state of the main chamber 10A can always be maintained to effect reduction in cost of the grinding operation.