To provide a volume reduction device for a thermoplastic polymer piece, which enables treated matter to be efficiently fed to an extruder.

In this volume reduction device 1 for the thermoplastic polymer piece, a force-feeding means 20 for forcedly feeding the thermoplastic polymer piece is provided in a feed passage 22 for the thermoplastic polymer piece which is continuously provided in a feed opening 11b of a screw type extruder 10; the feed passage 22 for the means 20 is installed in such a manner as to be tilted to the upstream side of the extruder 10 in the range of an angle of 10-70° with respect to the center axis of the extruder, in a side view wherein the feed passage 22 is directed upward.