In a disclosed image forming apparatus, an original is scanned and image data of the original are obtained. The data size of the image data corresponding to one page of the original is measured. The image data obtained are stored in or deleted from a memory. A storable number of pages of the original that can be stored in the memory is calculated based on the data size measured and available space in the memory. In response to the storable number of pages calculated being less than a total number of pages of the original, the image data of the original is successively stored in and deleted from the memory.