A seal ring is provided which allows the interior of a cylinder 1 of a screw extruder to be maintained at elevated temperature and elevated pressure, thus suppressing a variation in the pressure in the cylinder 1. In a special seal ring 300 attached to a screw shaft 7 in the cylinder of the screw extruder so as to be rotatable integrally with the screw shaft 7, thus restraining a material to be treated from being fed from upstream to downstream in the cylinder, a lead groove 317 extending in an axial direction of the shaft 7 is formed in an outer circumferential surface 316 opposing with a predetermined gap to an inner circumferential surface 1a of the cylinder. The lead groove 317 serves to disturb the flow of the material to be treated passing between the outer circumferential surface 316 and the inner wall surface 1a of the cylinder 1 to complex the flow. This stabilizes feeding resistance to suppress a variation in the pressure in the cylinder 1.