1. An image forming system, comprising: a plurality of image forming apparatuses that forms images onto a recording paper sheet; and a network through which the plurality of image forming apparatuses is serially cascaded so as to make it possible to share operations for forming the images onto different areas of the recording paper sheet between the plurality of image forming apparatuses; wherein each of the plurality of image forming apparatuses comprises: an image forming section that forms an image onto a paper sheet; and a control section that is provided with a process condition adjusting function for adjusting a process condition to be employed at a time when forming the image onto the paper sheet corresponding to a target density currently established, and a test-chart outputting function for outputting a test chart under the process condition above-adjusted; and wherein, corresponding to the target density, each of the plurality of image forming apparatuses, which are operated in conjunction with each other through the network, revises the process condition for a plural number of times within an adjustable range, and outputs the test chart at every time when the process condition is revised.