To provide both a method for reducing the volume of a foamed polystyrene and an apparatus therefor by which a distillation plant apparatus is not required and the amount of a solvent used and the cost required for heating are reduced as compared with those in conventional methods and apparatuses in relation to the recovery or treatment of wastes, especially the method for reducing the volume of various kinds of the foamed polystyrenes and the apparatus therefor.

This method for reducing the volume of the foamed polystyrene comprises a heating step of heating an animal or a vegetable oil at 200-230°C, a volume reducing step of scattering the heated animal or vegetable oil on the foamed polystyrene and reducing the volume of the foamed polystyrene, a storing step of storing the mixture of the foamed polystyrene reduced in the volume with the animal or vegetable oil, allowing the mixture to stand and precipitating the foamed polystyrene reduced in the volume and a separating step of separating the foamed polystyrene precipitated in the storing step and reduced in the volume from the animal or vegetable oil.