1. A copy machine comprising: an original reading apparatus which comprises a front surface image reading sensor and back surface image reading sensor, the original reading apparatus being capable of setting plural original documents, feeding out the original documents one by one, and simultaneously reading both surfaces of each of the original documents; an image forming apparatus for forming an image on a recording sheet based on image data obtained by the original reading apparatus; a post processing apparatus for executing post processing of at least one of stapling and punching onto the recording sheet on which the image is formed; a display section; a control section; an image processing section which rotates an image by 180 degrees with respect to the image data; an image memory; wherein, in a copying operation with the post processing being designated, the control section controls in such a way that: in cases where the memory capacity required for image rotation is available in the image memory, the original documents are read to obtain image data by the front surface image reading sensor; the obtained image data is subjected to the image rotation by the image processing section; and an image is formed on the recording sheet by the image forming apparatus based on image data having been subjected to the image rotation; whereas, in cases where the memory capacity required for the image rotation is not available in the image memory, an indication to re-set the original documents is displayed on the display section; the original documents having been re-set are fed out to be read by the back surface image reading sensor; an image is formed on the recording sheet by the image forming apparatus based on image data obtained by the back surface image reading sensor; and the post processing is executed onto the image-formed recording sheet, by the post processing apparatus.