In order to discharge electrical charges from a smoothing capacitor even if no discharge command is issued by a control system, making use of the displacement of a power unit in the event of a collision, a movable electrode is connected to a first contact connected to a high-potential bus, and the smoothing capacitor is connected with a converter and an inverter to be in parallel in the case where an engagement rod is positioned by a retaining ring. Further, when the engagement rod is disengaged from the retaining ring and removed from a relay, the movable electrode is held in contact with a second contact by a pressure spring, and the electrical charge remaining in the smoothing capacitor is discharged to a low-potential bus through a discharge portion.