An adapter is cylindrical and includes a first groove and a locking structure. The first groove is adjacent to an inner peripheral surface at one end of the adapter. The first groove extends in an axial direction of the adapter and turns in a circumferential direction. The locking structure is disposed in a circumferential direction at an outer peripheral surface at other end of the adapter. An optical plug includes an elastic member and a stopper. The elastic member is at a rear end of a holder. The stopper includes an annular portion that is in contact with the elastic member and arm portions bent toward the front end from an outer periphery of the annular portion. Each of the arm portions includes a hook at a front end of the stopper. The hook is configured to be locked into the adapter. The optical plug is attached via the adapter.