To prevent the end of a sliding portion of a male shaft from being fixed to a female shaft.

In the sliding portion 20, a portion 20a on the side of a projection 30 (fall-off prevention portion) of the female shaft 6 is exposed without being coated with a non-ferrous material (resin). Therefore, even if the male shaft 5 is pulled out of the female shaft 6 by a force, the exposed portion (skin portion) 20a is locked on the projection 30 (fall-off prevention portion) and there is no possibility that a portion coated with resin is locked on a fall-off prevention portion as a conventional one, so that the end of the sliding portion 20 can be effectively prevented from being pressed and fixed to the projection 30 (fall-off prevention portion).