A mechanism for winding camera film includes a small size reduction gear which includes an internal gear rotatably mounted within the camera, an output shaft mounted on the internal gear to rotate therewith, and a cylinder coupled to the output shaft to rotate with the output shaft for receiving and winding film thereabout, the internal gear being a stationary internal gear; a movable internal gear having an internal gear, plane gears in mesh with the stationary internal gear and the movable internal gear. A holder and a holder clamp hold the plane gears. A sun gear meshing with the planet gears is directly connected with a motor shaft. The reduction gear includes a column beam provided between columns extending upwardly from the holder and, further, guide sections are provided in those sections where the movable internal gear and the stationary internal gear are in contact with each other.