A sheet stacking unit of the disclosure includes a sheet stacking plate, a cursor, and a detection sensor. The cursor includes an alignment part for performing widthwise alignment of a document, and a rack extending along the widthwise direction. The detection sensor is placed opposite the rack, and has a light-emitting portion and a light-receiving portion. A plurality of detection-object parts placed in the rack along the widthwise direction have optical characteristics of different reflectivities or transmissivities from one another with respect to light emitted from the light-emitting portion. The controller detects a widthwise size of the document on a basis of output values from the detection sensor which detects light reflected or transmitted by the detection-object parts.