A belt tension adjustment device includes 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. The tension adjustment mechanism includes a support shaft rotatably supported, a rotational operation mechanism to rotate the support shaft about an axis of the support shaft, and 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.