In the related art, an image reading device determines a size of an original document using an image sensor used for reading the original document. The image sensor detects light emitted from a light source and reflected by the original document. Based on the intensity of the light detected by the image sensor, the image reading device determines the size of the original document.
However, when a platen cover is not closed, the plurality of sensors detects ambient light coming from outside of the image reading device. Hence, in some cases, the image reading device may not precisely determine the size of the original document.