An image reading device includes a memory unit for storing positional information of a sensor for moving by a drive unit to read an image of a document and a control unit for controlling the drive unit to move the sensor based on the positional information stored in the memory unit. The control unit drives the drive unit by an amount of driving sufficient for an image reading unit to move in a predetermined direction to abut the stop members, controls the drive unit to move the sensor so as to detect the position mark, and updates the positional information of the memory unit to a predetermined initial value based on the detection of the position mark by the sensor. The image reading device can store data of a reference position promptly even if a photo-interrupter is not provided when the data of the reference position is not stored in a memory unit.