An image reading apparatus includes a document sheet table, an image reading portion, a detection processing portion, and a determination processing portion. A document sheet is placed on the document sheet table. The image reading portion reads linear image data representing an image of a linear area that extends along a main scanning direction of the document sheet that is placed on the document sheet table. The detection processing portion detects, based on the linear image data, a size of the document sheet in the main scanning direction. The determination processing portion determines, when the size of the document sheet in the main scanning direction detected by the detection processing portion meets a specific condition, that the document sheet is a predetermined card-shaped document sheet.