backdoor 3 can be restrained according to the embodiment of the present invention the rubber ring 28 is provided between the driven gear 27 and the upper case 23 thus even when the direction of the urged rotational force affecting the driven gear 27 via the rod 13 and the arm 12 in accordance with the resultant force of the force of the damper stay 5 and the weight of the backdoor 3 is fluctuated the sudden change of the urged rotational direction can be restrained by the sliding resistance of the rubber ring 28 in addition because the movement of the driven gear 27 relative to the drive gear 25 within the backlash range is restrained the undesirable sudden swinging movement due to the sudden movement of the backdoor 3 can be restrained thus the opening and closing operation of the backdoor 3 can be performed smoothly according to the embodiment of the present invention the rubber ring 28 can be provided by using the existing housing i e upper case 23 for accommodating the driven gear 27 thus the burden at design change can be mitigated according to the embodiment of the present invention the rubber ring 28 is positioned approximately coaxial to the driven gear 27 this absorbs the movement of the driven gear 27 in the axial direction according to the embodiment of the present invention the movement within the range of the backlash can be swiftly performed during the initial stage by providing the rubber ring 28 for restraining the sudden change of the urged rotational direction in the drive unit 11 on a portion closest to a final deceleration portion of the drive unit 11 that is the loaded side i e backdoor 3 embodiment.