With a cover in a closed position, a first internal gear and a first spur gear do not mesh with each other, and a second internal gear and a second spur gear do not mesh with each other. When the cover rotates by a predetermined angle around a first support shaft from the closed position, the first internal gear and the first spur gear start to mesh with each other. When the cover rotates by a predetermined angle around a second support shaft from the closed position, the second internal gear and the second spur gear start to mesh with each other.