To provide a seal packing capable of preventing local increase of sealing contact pressure and capable of securing high sealing property by preventing partial wear.

The seal packing is arranged at an outer circumferential side of a stem 17 of a valve so that its cross-section shape becomes a substantially quadrilateral ring shape. Upper and lower surfaces are inclined surfaces D, E which are inclined in a same direction at a predetermined angle for a shaft direction of the stem 17. An inner circumferential surface is an inclined surface G having a clearance between an outer circumferential surface of the stem 17 and the inner circumferential surface when an axial pressing force is not applied. The inner circumferential surface is inclined to the stem 17 side and contacts with the outer circumferential surface of the stem 17 when an axial pressing force is applied.