The present invention includes: a first shaft member a rotating member rotatably supported by the first shaft member a movable member having a bearing and movably provided on the rotating member a second shaft member rotatably supported by the bearing a rotation transmitting unit which has an endless belt suspended between the first shaft member and the second shaft member, and which rotates the second shaft member with respect to the rotating member in a direction opposite the direction of rotation of the rotating member by an angle of rotation that is equal to an angle of rotation of the rotating member by transmitting the rotation of the rotating member to the second shaft member using the endless belt and a fixing unit which is capable of fixing the movable member to the rotating member. The fixing unit is capable of unfixing the movable member from the rotating member. By unfixing the movable member from the rotating member fixed by the fixing unit, the tension of the endless belt can be adjusted by changing the distance between the first shaft member and the second shaft member by moving the movable member in a predetermined direction. By fixing the movable member to the rotating member by the fixing unit, the distance between the first shaft member and the second shaft member can be maintained against the tension of the endless belt.La présente invention comprend : un premier élément darbre un élément rotatif soutenu de manière rotative par le premier élément darbre un élément mobile ayant un palier et disposé de manière mobile sur lélément rotatif un second élément darbre soutenu de manière rotative par le palier une unité de transmission de rotation qui a une courroie sans fin suspendue entre le premier élément darbre et le second élément darbre, et qui tourne le second élément darbre par rapport à lélément rotatif dans une direction opposée à la direction de rotation de lélément rotatif dun angle de rotation qui est égal à un angle de rotation de lélém
