a plug side surface of a plug housing is provided with a claw portion . the claw portion includes a plug lock surface facing in a direction away from a connector mounting surface . each assistant fitting of a receptacle connector includes a receptacle lock surface that faces in a direction approaching the connector mounting surface and is opposed to the plug lock surface in a mated state . the plug lock surface includes a lock maintaining surface and an unlocking surface . assuming that an angle formed between a reference plane and the lock maintaining surface is a lock maintaining angle and an angle formed between the reference plane and the unlocking surface is an unlocking angle , the lock maintaining angle is smaller than the unlocking angle .