An image reading device includes an accommodation section, an aligning member, a reading member, and a width detector. The accommodation section accommodates at least one document. The aligning member is provided in the accommodation section, is supported in a movable manner in a width direction intersecting a transport direction of the document, and aligns a widthwise edge of the document. The reading member reads an image of the document passing through a read position preset in a transport path. The width detector detects a width of the document based on a widthwise position of the aligning member if the document is of a typical size, and detects the width of the document based on the image read by the reading member if the document is of an atypical size.