Seal rings can keep the inside of a cylinder (1) of a screw extruder at high temperature and high pressure to suppress a variation in the pressure in the cylinder (1). Special seal rings (300) rotatably integrally mounted, in a cylinder (1) of a screw extruder, to screw shafts (7) and suppressing delivery of an object being processed from the upstream to downstream portions in the cylinder (1). Lead grooves (317) extending in the screw shafts (7) in the axial direction thereof are formed in outer peripheral surfaces (316) facing the inner peripheral surface (1a) of the cylinder (1) with a predetermined gap therebetween. The lead grooves (317) disturb the flow of the object being processed passing through the gap between the outer peripheral surfaces (316) and the inner peripheral surface (1a) of the cylinder (1), and this stabilizes delivery resistance to suppress a variation in the pressure in the cylinder (1).