To obtain a vol. reduction agent which quickly softens a polystyrene foam and reduces the vol. of the foam without dissolving it and without being adsorbed by it and which enables high-purity polystyrene to be easily recovered even in a small-size treatment by selecting an aliph. polyhydric alcohol as the agent.

The aliph. polyhydric alcohol used is one selected from among glycerin, polyethylene glycols including diethylene glycol and triethylene glycol, and trimethylolpropane. These alcohols softenes a polystyrene foam at a low temps., is not inflammable near at the vol. reduction temp. that is higher than the temp. for softening the foam, exhibits a low volatile loss, does not undergo the change in quality, and can be easily removed from polystyrene having the vol. reduced. Pref., 1-15 wt.% compd. selected from among an alkali metal carbonate, borate, sulfate, or phosphate, an alkali or alkaline earth metal chloride, and boric acid is added to the polyhydric alcohol, the main component of the vol. reduction agent.