To prevent lowering of seating ability and damage to a valve due to thermal expansion by properly absorbing thermal expansion of a valve stem.

A drive nut 21 joined in a screwed-in state and holding a valve stem 1 is indirectly supported to a valve housing 3 through first and second pressurizing springs 24 and 25, whereby expansion of the valve stem 1 during full opening is absorbed in such a way that the drive nut 21 is raised toward the valve housing 3 as the first pressurizing spring 24 is compressed. Expansion of the valve stem 1 during full closing is absorbed in such a way that the drive nut 21 is lowered toward the valve housing 3 as the second pressurizing spring 25 is stretched.