A belt device comprising: an endless moveable belt with a scale formed thereon along a circumference, the scale includes a plurality of marks; a first sensor (6, 70) that detects a first speed of the belt based on detection of the marks; a motor configured to drive the belt; and a first belt-speed controlling unit that controls a speed of the belt based on the difference between the first speed and a predetermined reference speed; characterised in that the device further comprises: a second sensor (65, 70') that detects a second speed of the belt based on a method other than detection of the marks; a second belt-speed controlling unit that controls a speed of the belt based on the difference between the second speed and the predetermined reference speed; a scale-mark degradation determining unit that determines whether there is a degradation in the quality of the marks, due to the marks being worn out, damaged or dirty, based on the signal from the sensor and detects an amount of degradation upon determining that there is a degradation in the quality of the marks based on a comparison of the signal from the sensor and a predetermined threshold value; and a belt drive controller that causes the first belt speed controlling unit to control a speed of the belt if the amount of degradation detected by the scale-mark degradation determining unit is less than a predetermined value, and causes the second belt speed controlling unit to control a speed of the belt if the amount of degradation detected by the scale-mark degradation determining unit is not less than the predetermined value. an endless moveable belt with a scale formed thereon along a circumference, the scale includes a plurality of marks; a first sensor (6, 70) that detects a first speed of the belt based on detection of the marks; a motor configured to drive the belt; and a first belt-speed controlling unit that controls a speed of the belt based on the difference between the first speed and a predetermined reference speed; characterised in that the device further comprises: a second sensor (65, 70') that detects a second speed of the belt based on a method other than detection of the marks; a second belt-speed controlling unit that controls a speed of the belt based on the difference between the second speed and the predetermined reference speed; a scale-mark degradation determining unit that determines whether there is a degradation in the quality of the marks, due to the marks being worn out, damaged or dirty, based on the signal from the sensor and detects an amount of degradation upon determining that there is a degradation in the quality of the marks based on a comparison of the signal from the sensor and a predetermined threshold value; and a belt drive controller that causes the first belt speed controlling unit to control a speed of the belt if the amount of degradation detected by the scale-mark degradation determining unit is less than a predetermined value, and causes the second belt speed controlling unit to control a speed of the belt if the amount of degradation detected by the scale-mark degradation determining unit is not less than the predetermined value. a second sensor (65, 70') that detects a second speed of the belt based on a method other than detection of the marks; a second belt-speed controlling unit that controls a speed of the belt based on the difference between the second speed and the predetermined reference speed; a scale-mark degradation determining unit that determines whether there is a degradation in the quality of the marks, due to the marks being worn out, damaged or dirty, based on the signal from the sensor and detects an amount of degradation upon determining that there is a degradation in the quality of the marks based on a comparison of the signal from the sensor and a predetermined threshold value; and a belt drive controller that causes the first belt speed controlling unit to control a speed of the belt if the amount of degradation detected by the scale-mark degradation determining unit is less than a predetermined value, and causes the second belt speed controlling unit to control a speed of the belt if the amount of degradation detected by the scale-mark degradation determining unit is not less than the predetermined value.