A volume reducing apparatus that is capable of eliminating wastage of an item to be housed and is capable of reducing the volume of a storage portion with a simple structure and without dirtying the hands of a user by the item to be housed, and a toner supplying apparatus that uses this volume reducing apparatus, which comprise a housing portion in which toner is housed; a shutter member; a cylinder linked to the housing portion; a piston freely movable inside the cylinder; and an air supply opening of a pump (medium discharge portion) that links to the cylinder and discharges air into the cylinder; wherein a toner discharge opening (opening portion) is formed at the housing portion, a linking hole is formed at the cylinder, and the piston has an air hole that links to the linking hole at another end side inside the cylinder, and the piston moves from one end side to the other end side inside the cylinder when air is discharged from the air supply opening into the cylinder such that the air hole of the piston and the linking hole of the cylinder link and air is discharged into the housing portion and the toner discharge opening is closed by the shutter member.