To securely lock connectors for charging, a charging connectors 1 is engaged with a vehicle-side connector 25, lock balls 9 are engaged with an engaging groove 28 and a locking member 10 is engaged with the lock balls 9. As a result, the connectors 1 and 25 are locked. Since the connectors 1 and 25 are locked by a ball lock mechanism taking advantage of the engagement of the lock balls 9, the connectors can be more securely locked even if the connectors and wires are heavy as compared to a prior art locking mechanism adopting an elastically deformable resin lance or engaging member and, therefore, a better reliability can be ensured.