An autoclave apparatus for treating material in the form of waste, the apparatus comprising: - a pressure vessel having at least one opening for feeding material into the pressure vessel; - a door for closing said opening of the pressure vessel; - pressure regulating means for generating a pressure inside the pressure vessel above or below that of the atmosphere; - agitation means for agitating material received in the pressure vessel, wherein the agitation means comprises a drum, which is rotatably arranged inside the pressure vessel so as to be rotatable in relation to the pressure vessel and which has an inner space for receiving material that is introduced into the pressure vessel via said opening of the pressure vessel, the drum having a cylindrical wall designed to prevent material received in the inner space of the drum from falling into the interspace between the drum and the pressure vessel as the drum rotates in relation to the pressure vessel, and one or several agitation blades mounted along the inner side of said cylindrical wall of the drum so as to act on material received in the inner space of the drum as the drum rotates in relation to the pressure vessel; - drive means for rotating the drum in relation to the pressure vessel; and - steam supply means for supplying steam into the inner space of the drum, characterized in: - that the drum has an open end projecting through said opening of the pressure vessel, said door being arranged to close the opening of the pressure vessel as well as the opening at this open end of the drum; and - that the door has a part arranged to extend into the opening at said open end of the drum when the door is in its closed position. - a pressure vessel having at least one opening for feeding material into the pressure vessel; - a door for closing said opening of the pressure vessel; - pressure regulating means for generating a pressure inside the pressure vessel above or below that of the atmosphere; - agitation means for agitating material received in the pressure vessel, wherein the agitation means comprises a drum, which is rotatably arranged inside the pressure vessel so as to be rotatable in relation to the pressure vessel and which has an inner space for receiving material that is introduced into the pressure vessel via said opening of the pressure vessel, the drum having a cylindrical wall designed to prevent material received in the inner space of the drum from falling into the interspace between the drum and the pressure vessel as the drum rotates in relation to the pressure vessel, and one or several agitation blades mounted along the inner side of said cylindrical wall of the drum so as to act on material received in the inner space of the drum as the drum rotates in relation to the pressure vessel; - drive means for rotating the drum in relation to the pressure vessel; and - steam supply means for supplying steam into the inner space of the drum, - that the drum has an open end projecting through said opening of the pressure vessel, said door being arranged to close the opening of the pressure vessel as well as the opening at this open end of the drum; and - that the door has a part arranged to extend into the opening at said open end of the drum when the door is in its closed position.