A scale including equally spaced marks is attached to a belt in a direction of movement of the belt such that a gap is formed between a first end and a second end of the scale. A first sensor and a second sensor detect the marks on the scale and output first signals and second signals respectively upon detecting the marks. The first sensor and the second sensor are located at different positions along the direction. A controlling unit controls the speed based on any one of the first signals and the second signals according to a position of the gap detected by the first sensor and the second sensor.