To provide an image scanner which is capable of discriminating the size of each manuscript.

In a state where a back plate 29 is arranged at a normal position or an upward position, the front edge of a manuscript stopped above a light transmitting plate 26 and the back plate 29 are read by an image pickup device by one line. That is, a reading operation is carried out before and after the position of the back plate is changed. As a result, when image data obtained before and after the position of the back plate is changed are compared with each other, the image data of the back plate 29 obtained before and after are different from each other. Therefore, the width of a manuscript can be discriminated based on the image data obtained before and after a position change in the back plate 29. Therefore, the size of each manuscript can be decided.