To provide the seal structure of a valve stem capable of facilitating the exchange of a shaft seal member without leakage of water in a state that pressure water flows inside a valve casing and a pipe line in which the valve casing is interposed.

A gland part of a stuffing box 8 is filled with a shaft seal member 4 consisting of a gland packing group, and the shaft is sealed with the shaft seal member 4 axially pressed by a packing presser 6. The tip part of an annular press part 8A protruded from the tip part of the stuffing box 8 is brought into contact with an elastic seal ring 10, and after the elastic ring 10 is elasticity deformed by increasing the fastening force of a second fastening mechanism 12 and held in a shaft seal state to force the seal ring into pressure contacted to the valve stem 5, the shaft seal member 4 is exchanged.