An arm (20) facing a male member (10) is provided with a claw (30) protruding toward the male member (10). An adapter (1) can be connected to a female connector while the male member is inserted in the female connector and the claw is engaged with the female connector. The arm can elastically deform so that the claw separates from the male member. The claw is provided with a sliding section (33a) which slides on the female connector during the connection of the adapter to the female connector. The sliding section moves the claw in the direction away from a male connector when the sliding section slides on the female connector. The contour shape of the sliding section when viewed in the direction perpendicular to the center axis (1a) passing through the male member includes a smooth convex curve.
