the first image formation mode is a mode for outputting image data scanned from one original to one sheet and the second image formation mode is a mode for reducing image data scanned from plural originals and outputting the image data to one sheet a so called 2 in 1 mode or 4 in 1 mode the cpu 101 prohibits image formation in the second image formation mode when the external storage device 200 is designated 5 the image forming apparatus according to this embodiment prohibits an image formation mode for scanning and printing left and right pages of a book with the left and the right pages divided for example when the external storage device 200 is designated consequently it is possible to prevent an error due to a shortage of a capacity of the external storage device 200 in advance the printer unit 110 forms an image in a designated mode of first and second image formation modes the first image formation mode is a mode for forming while scanning an image with the scanner unit 104 an image on the basis of image data obtained by this scanning and not requiring storage of the obtained image data after.