A speed reducer provided with a gear case is mounted onto an electric motor, and a pinion is provided on an output shaft projecting from the gear case. A radially projecting base part is provided on the gear case, and a support shaft is attached to the base part. A sector gear is fixed to the base end of a lift arm, and the sector gear is so supported by the support pin as to be capable of swinging and meshed with the pinion. An arcuate groove part is formed in the sector gear. A gear cover is fixed to the gear case by a bolt extending through the groove part and a bolt disposed on further outer peripheral side than the sector gear. The gear cover covers the meshed portions of the pinion and the sector gear while pressing the sector gear against the gear case.