A sheet sorting apparatus includes a first tray detachable from an apparatus main body, a second tray arranged below the first tray, a discharge unit to discharge a sheet to the first or second tray, a tray detecting unit to detect that the first tray is detached from the apparatus main body, a full stack detecting unit to detect a fully stacked condition of the first tray, and a control unit to control the discharge unit. In a case where the tray detecting unit detects that the first tray is detached from the apparatus main body, the control unit controls the discharge unit not to discharge a sheet to the second tray when a number of sheets discharged to the second tray reaches a threshold number of sheets, even though the full stack detecting unit does not detect the fully stacked condition.