The present invention provides a sheet feeder apparatus in which a regulating member can securely be fixed without generating lateral slip of a sheet and an image forming apparatus. The regulating member regulates a sheet position in a width direction, and a fixing portion fixes the regulating member to a position according to a sheet size. The fixing portion includes plural rack gears which are provided in a surface opposite a surface abutting on the sheet of the regulating member; plural relay gears which engage the plural rack gears; a rotating shaft on which the plural relay gears are provided; and a lock mechanism which regulates rotation of the rotating shaft when the regulating member is moved away from the regulated sheet, the lock mechanism permitting the rotation of the rotating shaft when the regulating member is moved in a direction in which the regulating member contacts the regulated sheet.