An auxiliary housing (20) inserted into a frame (10) is locked in a retained state by the locking action of resiliently deformable lock arms (13) and locks (21). The lock arms (13) and the locks (21) include locking surfaces (17, 23) that are substantially normal to an inserting direction of the auxiliary housing (20) and come into contact with each other with the lock arms (13) and the locks (21) engaged in an insertion process of the auxiliary housing (20). When an inserting force exceeding a locking force of the locking surfaces (17, 23) is applied to the auxiliary housing (20), the lock arms (13) are resiliently deformed to disengage the locking surfaces (17, 23).