PURPOSE: To detect wear of a screwed surface automatically by screwing a valve shaft down on a drive sleeve to allow reciprocal driving in the direction of an axis line while a dummy ring is screwed down having a freedom in the direction of the axis line of the drive sleeve to provide a detection circuit on the dummy ring.

CONSTITUTION: A thread part 2a is screwed down on the circumferential surface of a valve shaft 2 to match a thread part 1a of a drive sleeve 1. The material of the drive sleeve 1 is lower in hardness than the material of the valve shaft 2, so that the thread part 1a of the drive sleeve 1 wears earlier than the thread part 2a of the valve shaft 2. When a matching of a coil is made by adjusting a distance between a dummy ring 14 and a mounting base 9 on the output side, an induction current flows through a detection coil of a detection circuit 7 of the dummy ring 14. When a screwed surface wears, a clearance δ occurs between the drive sleeve 1 and the valve shaft 2. As the impedance on the side of the detection coil changes according to the degree of the clearance δ, the change in the impedance is sensed on the side of the output coil.