A flexible arm with a locking ramp is mounted on a plug connector in a resiliently swingable manner. The locking ramp abuts against a locking latch formed on the receptacle connector when the plug unit is inserted in the socket hole formed in a receptacle. Further pressing of the plug unit causes the locking latch to press down the locking ramp so that the plug and receptacle units are perfectly disengaged from each other until the peak of the locking ramp exceeds the bottom of the locking latch. Thus, the connector is forcibly and automatically re-set at a pre-engagement position where the terminals are non-conductive when the connector is not completely engaged.