When a polyethylene bag with medical wastes received therein is supplied to a cylindrical-shaped housing and a compression device is used to lower a piston to compress the medical wastes, the piston is repeatedly caused to go up and down to accomplish stepwise compression whereby the polyethylene bag with medical wastes received therein is prevented from entering into a clearance between an inner wall of the cylindrical-shaped housing and the piston and from bursting due to rapid compression, and when cake obtained by melt sterilization and volume reduction with heating compression is discharged, a gate is fully opened to lower a lower surface of a heating device of the piston to a point distant several millimeters from an upper surface of a heating device of the gate to enable forcedly scraping off the cake adhering to the heating device tightly joined to the piston upon closing of the gate.
