An image scanning apparatus of this disclosure includes: a scanner unit which generates image data by image scanning and outputs it; a memory; a data processing unit which (a1) performs a predetermined process for either the image data outputted by the scanner unit or the image data for which a preprocess has been performed, (a2) writes the processed image data into a predetermined memory area in the memory in turn, and (a3) returns a writing position of the image data to a top of the memory area and overwrite the image data if the writing position of the image data reaches an end of the memory area; and a control unit which (b1) allocates the memory area in the memory and (b2) measures a shortage amount of the memory area on the basis of a size of an area where the image data has been overwritten.