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 low scanning speed, the document tray is elevated one time until the upper surface height becomes a target height which is higher by a predetermined distance since a height detection sensor is turned on at an inter-transfer time between completion of document transfer and start of a subsequent document transfer. In an intermediate scanning speed, the document tray is elevated little by little in each transfer of one document at the inter-transfer time until the upper surface height reaches the target height. In a fast scanning speed, the transfer of the document is tentatively interrupted, the document tray is elevated one time until the upper surface height reaches the target height, and the transfer of the document is resumed.