A rotating-body restraining device includes a first gear provided on a rotational shaft of a rotating body and rotated together with the rotating body, a second gear that meshes with the first gear, a third gear provided on a rotational shaft of the second gear and rotated together with the second gear, and a restraining member that is movable toward the third gear and that restrains the third gear from rotating by moving to a position where the restraining member meshes with the third gear. The numbers of teeth included in the first, second, and third gears are Za, Zb, and Zc, respectively. Zb is not equal to the product of Za and an integer n. Zc is an integral multiple of a value obtained by dividing the least common multiple of Za and Zb by Za.