To improve work efficiency by performing a warning or changing a reading operation resulting in elimination of unnecessary operation when the size of an image to be read is larger than the size of an image memory where image information is stored in an image reader which reads an original image while carrying the original.

Image sensors 4 and 9 read images on the surface and rear face of an original at every line and stores them in an image memory 26. In such a case, the width and length of the original are detected to calculate image information quantity, and when the size of the image information quantity of the read image is larger than the capacity size of the memory 26, warning display is performed. Also, image reading only for the capacity of the memory 16 is performed as necessary, and the image information is transferred to an external host computer, etc.