To provide a volume reduction apparatus for polystyrene foam, or polyurethane foam, with an air carring apparatus, with which efficiency decrease of volume reduction treatment is prevented by controlling the increase of a pressure caused by a temperature rise in the treatment part of the volume reduction apparatus.

In this volume reduction apparatus having constitution that a material whose volume is to be reduced ground into a fixed size by a grinding apparatus 2 is sent by air from the grinding apparatus 2 through a transportation pipe 4 to a treatment part 82, dissolved to reduce the volume by the treatment part 82 and air having contributed to the transportation by air of the material whose volume is to be reduced is returned through a return pipe 6 to the grinding apparatus 2 again, the volume reduction apparatus is equipped with an internal pressure buildup controlling means for controlling the internal pressure buildup of the treatment part 82. The internal pressure buildup controlling means is constitued of an air vent hose 9 connected between the treatment part 82 and the grinding apparatus 2.