A gear device includes: an input gear fixed to an output shaft of a motor; an internal gear; an external gear which rotates while revolving around a central axis of the internal gear while meshing with the internal gear; a carrier supported rotatably within the internal gear and adapted to rotate about the central axis of the internal gear in conjunction with the rotation of the external gear; a plurality of crankshafts which are rotatably supported by the carrier and are adapted to cause the external gear to revolve around the central axis of the internal gear; transmission gears each provided on a central portion of each of the crankshafts; and a cylindrical gear which meshes with the transmission gears of the crankshafts and to which rotation is transmitted from the input gear, wherein the cylindrical gear is located on a radially inner side of an imaginary line passing through a center of rotation of each crankshaft and is rotatably supported by the carrier, and wherein the input gear and the cylindrical gear mesh with each other in an axially central portion of the carrier.