A belt tension adjustment device comprising: a driving pulley attached to a power transmission shaft; a driven pulley arranged in a driven device; a tension pulley; a loop belt arranged between the driving pulley, the driven pulley, and the tension pulley, and configured to transmit a rotational driving force of the power transmission shaft to the driven device; and a tension adjustment mechanism to adjust tension of the loop belt, a support shaft rotatably supported; a rotational operation mechanism to rotate the support shaft about an axis of the support shaft; a rotating member coupling the support shaft and the tension pulley, configured to change a relative position of the tension pulley with respect to the driving pulley and the driven pulley in accordance with rotating of the support shaft; and the rotating member includes: a first portion attached to one end of the support shaft; and a second portion which rotatably supports a rotation shaft of the tension pulley, characterized in that the tension adjustment mechanism further includes: a bracket having a fixed portion fixed to the driven device and a supporting portion to rotatably support the support shaft, wherein the rotational operation mechanism includes: a movable member attached to the support shaft; an operating portion to rotate the support shaft when the movable member is rotated; and a limiter mechanism to limit a range of rotation of the movable member, the movable member is attached to the other end of the support shaft, the support shaft is parallel to the rotation shaft of the tension pulley, having: the one end arranged on a side of the tension pulley; and the other end arranged on another side opposed to the side of the tension pulley, the operating portion is arranged on a side of the other end of the support shaft, and the limiter mechanism includes: a guide hole formed in the movable member, having an arc shape formed about the support shaft; and an inserting portion arranged in the bracket, configured to be inserted in the guide hole and to move in the guide hole in accordance with rotation of the movable member. a driving pulley attached to a power transmission shaft; a driven pulley arranged in a driven device; a tension pulley; a loop belt arranged between the driving pulley, the driven pulley, and the tension pulley, and configured to transmit a rotational driving force of the power transmission shaft to the driven device; and a tension adjustment mechanism to adjust tension of the loop belt, a support shaft rotatably supported; a rotational operation mechanism to rotate the support shaft about an axis of the support shaft; a rotating member coupling the support shaft and the tension pulley, configured to change a relative position of the tension pulley with respect to the driving pulley and the driven pulley in accordance with rotating of the support shaft; and a support shaft rotatably supported; a rotational operation mechanism to rotate the support shaft about an axis of the support shaft; a rotating member coupling the support shaft and the tension pulley, configured to change a relative position of the tension pulley with respect to the driving pulley and the driven pulley in accordance with rotating of the support shaft; and the rotating member includes: a first portion attached to one end of the support shaft; and a second portion which rotatably supports a rotation shaft of the tension pulley, characterized in that the tension adjustment mechanism further includes: a bracket having a fixed portion fixed to the driven device and a supporting portion to rotatably support the support shaft, wherein the rotational operation mechanism includes: a movable member attached to the support shaft; an operating portion to rotate the support shaft when the movable member is rotated; and a limiter mechanism to limit a range of rotation of the movable member, a first portion attached to one end of the support shaft; and a second portion which rotatably supports a rotation shaft of the tension pulley, characterized in that the tension adjustment mechanism further includes: a bracket having a fixed portion fixed to the driven device and a supporting portion to rotatably support the support shaft, wherein the rotational operation mechanism includes: a movable member attached to the support shaft; an operating portion to rotate the support shaft when the movable member is rotated; and a limiter mechanism to limit a range of rotation of the movable member, the movable member is attached to the other end of the support shaft, the support shaft is parallel to the rotation shaft of the tension pulley, having: the one end arranged on a side of the tension pulley; and the other end arranged on another side opposed to the side of the tension pulley, the one end arranged on a side of the tension pulley; and the other end arranged on another side opposed to the side of the tension pulley, the operating portion is arranged on a side of the other end of the support shaft, and the limiter mechanism includes: a guide hole formed in the movable member, having an arc shape formed about the support shaft; and an inserting portion arranged in the bracket, configured to be inserted in the guide hole and to move in the guide hole in accordance with rotation of the movable member. a guide hole formed in the movable member, having an arc shape formed about the support shaft; and an inserting portion arranged in the bracket, configured to be inserted in the guide hole and to move in the guide hole in accordance with rotation of the movable member.