A gear transmission device includes a first external gear driven by a drive motor attached to a first side of a frame and a first internal gear driven by the first external gear. The first internal gear includes an outer cylinder portion, an end plate, an annular opening, and an inner cylinder portion including a shaft hole,. The outer cylinder portion includes internal teeth formed on an inner circumferential surface thereof. The inner cylinder portion is formed concentrically inside the outer cylinder portion. The end plate connects the outer cylinder portion, the inner cylinder portion, and the shaft hole at one end in an axial direction. In a state in which the annular opening faces a second side of the frame opposite the first side thereof, the first external gear is inserted to the annular opening to mesh with the internal teeth of the outer cylinder portion.