An image reading apparatus that improves reading productivity during simultaneous double-sided reading without leading to increases in the size and cost of the apparatus and a method for controlling the image reading apparatus are provided. To accomplish this, the image reading apparatus, in the case of a single-sided reading mode, controls the scanning speed of a first image reading unit to a first scanning speed and controls the distance between original document sheets to a first distance between original document sheets. On the other hand, in the case of a double-sided reading mode, the image reading apparatus controls the scanning speed of the first image reading unit to a second scanning speed that is the scanning speed of a second image reading unit and that is slower than the first scanning speed, and controls the distance to a second distance that is shorter than the first distance.