An image forming apparatus having an automatic document feeding unit so as to successively read documents is disclosed. The image forming apparatus includes an image forming unit to form an image on a printing medium, an image reading unit pivotally coupled to an upper portion of the image forming unit to expose and shield a top of the image forming unit, an automatic document feeding unit pivotally coupled to an upper portion of the image reading unit to expose and shield a top of the image reading unit, and a locking device to selectively expose any one of the top of the image forming unit and the top of the image reading unit. Since the locking device prevents the top of the image forming unit and the top of the image reading unit from being simultaneously exposed, the automatic document feeding unit can be prevented from being damaged due to the simultaneous exposure of the top of the image forming unit and the top of the image reading unit.