An image reading apparatus includes a read size determination circuit. The read size determination circuit has a plurality of operation mode including a first mode and a second mode. The first mode determines the size of the document detected by the first document reading unit as a first read size and determines a read size of the document set in advance as a second read size. The first read size is the read size of the document when the first document reading unit reads the document. The second mode determines the size of the document detected by the first document reading unit as the first read size. If the second document reading unit reads the document, the second mode determines the read size of the document set in the setting screen as the second read size when a start instruction of a reading job is entered using the operation circuit.