There is provided an automatic document feeder, comprising: a document tray; an output tray; a movable ejection flap configured to be movable between a first position where the document for which the image reading is finished is sunk into a position between the output tray and a bottom of a stacked document already staked on the output tray and a second position where the document is ejected on the output tray without being sunk into the position; a setting unit configured to set the movable flap to one of the first position and the second position; a change unit configured to change a position of the movable flap to the second position in response to a fact that a predetermined condition is satisfied in a state where the position of the movable flap has been set to the first position through the setting unit.