To provide an image forming system having an inserter device easy of installation work, without enlarging a postprocessing device.

This image forming system is provided with an image forming device for forming an image on paper based on input image information and the postprocessing device 3 for postprocessing the image-formed paper, and is provided with the removable inserter device 4 capable of separately supplying the paper to a carrying passage for carrying the paper to the postprocessing device 3 from the image forming device, between the image forming device and the postprocessing device 3. The image forming system is formed by arranging a locking object part 16 in the postprocessing device 3, and arranging a locking part 17 lockable from above on its locking object part 16 and lockable on the locking object part 16 so as to be mutually relatively horizontally freely movable.