Two connectors (20, 40) are separated until an initially connected state is reached by rotating a nut (70) in a loosening direction to release the nut (70) from a threadedly engaged state in a properly connected state of the two connectors (20, 40), and an interlock circuit is turned off during this time to set a power supply circuit to a cut-off state. A partial locking mechanism (80) is provided between the two connectors (20, 40) for preventing any further separation by locking the power-supply side connector (40) when the power-supply side connector (40) is separated up to an initial connection position and enabling the power-supply side connector (40) to be completely separated when a specified force or larger is applied to the power-supply side connector (40).