An optical ruler is provided on the back surface side of the top housing and close to the transparent window along a scanning direction for providing a calibration reference. The optical ruler is formed with black and white blocks alternately arranged in an array. An image sensor which has a width larger than the width of the transparent window is provided for reading the image information of the optical ruler along with a scanned sheet while being progressively driven by a step motor. A determining element computes the actual moving steps of the step motor by calculating the number of black and white pixels from the scanned image of the optical ruler. When the actual moving steps of the step motor are fewer than a predetermined threshold value, or when the frequency of the occurrences of the scan line losses is higher than a predetermined threshold value, the image sensor is driven by the step motor to scan the sheet again. As a result, the step motor can be operated in response to a closed loop control signal depending on the actual moving steps of the step motor.