A structure for supporting a pulley holder is provided with a pulley holder for rotatably supporting a pulley around which a belt is movably wound, a frame member that slidably supports the pulley holder along the direction of stretching the belt, and an urging member that elastically urges the pulley holder in a belt tensioning direction in which the belt is tensioned. The pulley holder has fitting portion that is stepwise formed along the belt tensioning direction, and the frame member has a fitting opening whose edge is stepwise formed corresponding to the fitting portion.