To prevent a fall of a gland packing during additional tightening by a method wherein a fall preventing part fitting in a small part to support a secondary gland packing is arranged at the end face on one end side of a spring holder positioned opposite to a small part formed on the inner circumferential surface of a stem boss part arranged in a state to be fitted externally in a stem.

In the shaft seal part of a high temperature slide valve, a spring holder 21 comprises a drum part 22 on which a spring 10 is mounted on the side corresponding to a bush 9; and a fall preventing part 23 protruded in the axial direction of a stem 2 from an end face on one end side positioned opposite to a small part. A secondary gland packing 12 is supported at the tip of the fall preventing part 23. Further, when a packing gland 17 is pressed in the axial direction of the stem 2 by a tightening bolt 18 and primary and secondary gland packings 8 and 12 are additionally tightened, the spring holder 9 is also moved in the axial direction of the stem 2. But, since the fall preventing part 23 is all the time in a state to be fitted in the small part, a fall of the secondary gland packing 12 from the periphery of the stem 2 is prevented from occurring.