On the basis of first read image data obtained by reading a first face, a controller calculates a first sub scanning size, which is a size of the first face in a sub scanning direction. On the basis of second read image data obtained by reading a second face, the controller calculates a second sub scanning size, which is a size of the second face in the sub scanning direction. The controller determines the larger of the first sub-scanning size and the second sub-scanning direction size to a definitive sub scanning size, which is the size of the document in the sub scanning direction set on the document stage.