In a standby state wherein a document is set in a document tray and can be supplied when needed, when a start signal is not inputted (S21: NO), a controller determines whether a document regulating plate has been operated or not based on a detection output of a document size detector for detecting a position of the document regulating plate (S31), and when the document regulating plate was operated (S31: YES), the controller lowers the document tray by a predetermined amount and stops it (S32). A lowered amount of the document tray can be arbitrarily set by a user through an operator control panel. With a sheet feeding apparatus, an image reading apparatus and an image forming apparatus constituted as described above, even in a state in which the document is set in the document tray, the set state can be easily canceled by operating the document regulating plate.