PURPOSE: To detect wear of a screwed surface with a valve shaft of a drive sleeve automatically.

CONSTITUTION: A wear sensor 4 having a detector section 4b at the tip part thereof is mounted on a drive sleeve 1 so that the tip of the detector section 4b matches a screwed surface 1d with a valve shaft 2 while a detection coil 6 is provided to form a detection circuit 7 with the detector section 4b of the wear sensor 4 serving as switch. On the other hand, an output coil 10 is provided on the side of a valve case 8 to face the detection coil 6 while an output circuit 11 containing the output coil 10 is formed. As the screwed surface 1d of the drive sleeve 1 wears, the detector section 4b of the wear sensor 4 wears, causing a change in the impedance of the detection circuit 7. When with the wearing thereof, the detector section 4b vanishes completely, the impedance of the output circuit 11 changes. The changes of the impedance are outputted as changes in voltage or the like, thereby enabling detection of the wearing condition of the drive sleeve 1 automatically.