In an image scanning apparatus, when documents on a document tray are sequentially transferred, different control steps are performed according to a scanning speed when an upper surface height of the documents is lowered thereby turning off a height detection sensor. In a first control mode, which is not a high speed scanning, the document tray is elevated little by little during an inter-transfer time between each time one document is transferred until the upper surface height reaches a target height that is higher by a predetermined distance since the height detection sensor is turned on. In a second control mode, in which high speed scanning is performed, the document tray is elevated little by little during the inter-transfer time each time the one document is transferred until the upper surface height reaches a height that is higher by a predetermined distance than a height at a time when the height detection sensor is turned off.