An image reading apparatus includes a feeder, a first reading unit, a first memory which stores image data of the surface of the document read by the first reading unit, a second reading unit located at an upstream side of the first reading position, a second memory which stores image data of the document read by the second reading unit and a controller configured to control the feeder, the first reading unit and the second reading unit. The controller controls to execute pre-scanning the document by the second reading unit over a predetermined range shorter than a separation distance between the first reading position and the second reading position in a transporting direction from a leading edge of the document when the document reaches the second reading position. The controller further controls to execute storing obtained pre-scanned image data into the second memory and measuring the size of the document based on the pre-scanned image data stored in the second memory. The controller still further controls to execute reading the surface of the document by the first reading unit when the document reaches the first reading position, and storing obtained first image data into the first memory. The controller controls to execute processing the first image data stored in the first memory based on the size of the document obtained in the measuring.