A stopper (67) of a detector (60) is lockable to a lock surface (17) by entering a lock hole (18) of a lock arm (13) from a deflection space 15. A locking protrusion (45) of a second housing (40) locks to the lock surface (17) by entering the lock hole (18) from a side opposite the deflection space (15). Thus, the housings (10, 40) are held together and the stopper (67) pressed by the locking protrusion (45) separates from the lock surface (17) and the detector can move to the detection position. The locking protrusion (45) has a main body (46) wider than the lock surface (17) of a lock projection (16) and lockable to the lock surface (17). A pressing protrusion (47) narrower than the locking main body 46 projects from the locking main body (46) and enters the lock hole (18) to press the stopper (67).