To provide an image forming system for realizing a duplicating function inexpensively and rapidly by adding an image scanner on the image forming device to expand the function of the image forming device.

The image forming system 1 is constituted as follows. The image scanner 20 for scanning an image of a manuscript to output the image data is added on to the image forming device 10 for recording and outputting the image on a paper based on input image data to expand the function of the image forming device 10 and the image data of the manuscript scanned by the image scanner 20 are sent to the image forming device 10 to record and output the image on the paper by the image forming device 10. When realizing the duplicating function like this, the image scanner 20 has an image data storing function for storing the image data and sends the image data to the image forming device 10 after storing once the scanned image data of the manuscript by the image data storing function. Consequently, the image scanner 20 can be inexpensively added on to the image forming device 10, thus permitting the image forming system 1 whose function has been expanded by adding on the duplicating function or the like to exert high-speed processing ability and good availableness.