In a U-cup based seal of a rod, the basic body of the U-cup 3 is recessed relative to a square cross-sectional shape in the region of a radially inner outer edge 18 on the low pressure side in order to obtain a space 17 between the U-cup 3 and the opening, i.e. the passage opening 16 of a sealing gap 15, leading into a rectangular groove 4 guiding the U-cup 3. This space 17 remains at least partially also upon pressurization such that the U-cup 3 completely remains in the groove 4 upon pressurization and is not extruded into the sealing gap 15. This considerably reduces wear of the U-cup 3.