This image reading device includes an image sensor module, an environment maintaining portion, and a level correction portion. The image sensor module includes a light emitting portion and a plurality of single-channel light amount sensors. The environment maintaining portion maintains, at a constant state, a light receiving environment for a reference portion forming an end portion, of a single-channel light amount sensor, which corresponds to an end portion of the main scanning region. The level correction portion corrects, each time the image sensor module outputs detection data that corresponds to the main scanning region, a level of the detection data from each of all the single-channel light amount sensors in accordance with a deviation between a level of the detection data regarding the reference portion and a reference level set in advance.