A device for locking a pair of connectors to each other, in which a locking unit which is attached to the housing of one of the connectors in a pivotable manner, and is biased by a spring in a direction tending to unlock the connectors from each other. The locking unit includes a pair of support portions and a plate spring engagement portion. Support members are provided on the housing for receiving the support portion and a plate spring is resiliently disposed between the engagement portion and the housing.