An aspect of the invention provides an image scanner which can continuously transfer image data of one surface to an external device (image forming apparatus) without performing control such as interruption and resumption of an original scanning operation. The image scanner includes a size detection unit which detects an original size and a determination unit which determines whether or not the image data relating to an original of the size detected by the size detection unit can be stored in a storage unit. The image scanner also includes a control unit. The control unit is configured to set operation in a first mode when the determination unit determines that image data relating to the original can be stored in the storage unit, and the control unit is configured to set operation in a second mode when the determination unit determines that image data relating to the original cannot be stored in the storage unit.