An arm (20) that faces a male member (10) is provided with a claw (30) that protrudes toward the male member (10). An adapter (1) can be connected to a female connector with the male member being inserted into the female connector and the claw engaging with the female connector. The arm is elastically deformable so that the claw can move away from the male member. The claw includes a slidable portion (33a) that slides on the female connector during a process in which the adapter is connected to the female connector. The slidable portion moves the claw in a direction away from the male connector while the slidable portion slides on the female connector. A contour shape of the slidable portion seen in a direction that is orthogonal to a central axis (1a) that passes through the male member includes a smooth convex curve.