PURPOSE: To thermally shrink a high-bulkiness waste plastic, such as an expanded styrol, that is easily affected by a static electricity and difficult to process for the continuous volume reduction thereof without a trouble by providing a hot air generator for feeding hot air into a trough.

CONSTITUTION: Hot air is generated by a hot air generator 6 to increase a temperature in a trough 3 to approximately 150-180°C. The hot air is continuously fed into the trough 3 over the full length thereof. In this state, a ground material of a collected waste plastic, not limited to a high-quality plastic, is loaded into the trough 3 from an inlet 1. The ground material is moved in the trough 3 from its front end to its rear end by rotating a shaft-free spiral 4. As a result, a waste plastic, such as an expanded styrol, can be reduced in volume to have a density of substantially 0.03 to below 1.0.