An image scanning device includes a document table; a document pressing unit configured to press a document placed on the document table; a scanning unit including a light source for illuminating the document and configured to optically scan the document; an open/close detection unit configured to detect opening and closing of the document pressing unit; and a control unit configured to cause the scanning unit to move parallel to the document and thereby to detect a size of the document when the closing of the document pressing unit is detected by the open/close detection unit or when a scanning start signal is detected. The control unit detects the size of the document within a document size detection period that is changeable according to a speed of closing the document pressing unit, and turns on and off the light source based on the document size detection period.