A transmission device that transmits rotation of a drive cable to a screw shaft is provided. The transmission device includes a worm gear attached to the drive cable, a helical gear attached to the screw shaft, a gear case, a pair of first bearing wall portions, and a pair of second bearing wall portions. The gear case has an opening on one side so as to accommodate the worm gear and the helical gear. The first bearing wall portions are integrally formed with the gear case so as to face each other along the axial direction of the drive cable, and rotatably support both ends of the worm gear. The second bearing wall portions are integrally formed with the gear case so as to face each other along the axial direction of the screw shaft, and rotatably support both ends of the helical gear.