A disclosed image scanning device includes a pressing unit for pressing a document; an operation detection unit for detecting an opening operation and a closing operation of the pressing unit; a movable scanning unit for scanning the document; and a control unit for moving the movable scanning unit from a standby position to a document-size detection start position when the opening operation is detected and moving the movable scanning unit from the document-size detection start position to the standby position to determine a size of the document when the closing operation is detected. The control unit is configured to continue moving the movable scanning unit from the standby position to the document-size detection start position until the arrival of the movable scanning unit at the document-size detection start position is detected even if the pressing unit is operated.