Abstract: A locking device (20) is placed on an upper surface of housing (2) of connector (1) in a temporary retained state. The locking device (20) has a pair of beams (78) extending downward obliquely to each other and having notches (78a) in surfaces facing each other. The notches have shoulders which engage against a wedge-shaped rib (90) of the housing (2); the shape of the notches (78a) corresponds to the shape of the rib (90). Therefore, the locking device (20) cannot be inserted in the housing even by force. When mating connector (200) is coupled with connector (1), rib (230) spreads the beams (78) releasing the engagement between the notches (78a) and the rib (90), thus making it possible to insert the locking device (20) to a fully-engaged position.