To provide an inexpensive volume reducing auxiliary implement having an extremely simple shape in the volume reducing treatment for a PET bottle using vacuum suction and to reduce the volume of the PET bottle simply/easily using a household cleaner.

This volume reducing auxiliary implement 10 for the PET bottle is formed into the straight part 11 fitted to the suction nozzle 40 of a vacuum cleaner on its one end side and also formed into the cylindrical body, which can be inserted in the mouth part 55 of the PET bottle 50 on its other end side. The other end side is made tapered so as to become small in outer diameter toward the leading end of the volume reducing auxiliary implement. A moisture removing filter (not shown in a drawing) is formed by laminating filter cloths up to a thickness of about 2 mm and both main surfaces of the filter is held by stainless steel nets (having square openings of about 3 mm) to be attached to the end surface side of the tapered part 12.