In response to a reading start instruction, a second size detection processing portion detects a predetermined maximum sheet size for which reading can be performed by an image reading portion, among sheet sizes having the same size in a main scanning direction as that of a document sheet indicated by an output signal from a document sheet width sensor. A first blank sheet determination processing portion sets a first determination region corresponding to the sheet size, and a control portion causes the image reading portion to perform a reading operation. If image information of the first determination region is obtained, the first blank sheet determination processing portion determines whether a document sheet image is a blank sheet image, based on the image information. If the document sheet image is a blank sheet image, a first image processing portion deletes image information of the document sheet image.