A belt device includes a belt as an endless belt that has a scale with a large number of scale marks formed along the whole circumference of the belt. A sensor reads the scale to obtain scale information. A control device provides control to correct a belt speed, based on scale information read. A scale-mark degradation determining unit determines whether the scale mark is degraded. A belt drive controller continuously provides control to correct the belt speed for a degraded portion until the scale-mark degradation determining unit determines that there is no degradation.