Abstract: In a cross-section perpendicular to the extending direction of a support part, a first bar has a holding part that holds a belt together with a leading end part and a winding part on which the belt is wound. A third bar is positioned on one side of a virtual line ? connecting a point at the holding part at which the belt is held together with the leading end part of a moving part and a point at the support part of the moving part before being moved. An operating part of the moving part extends from the support part to the other side relative to the virtual line ?, and a biasing part of the moving part generates biasing force in such a direction that the leading end part is moved toward the first bar.