1. A method for controlling gloss difference in a plurality of image forming apparatus connected to a network, said method comprising steps of: forming a predetermined image on a sheet in each of the plurality of image forming apparatuses; measuring a glossiness of the image formed on each sheet; and controlling gloss difference among the plurality of image forming apparatuses by changing a gloss adjustment parameter of each of the plurality of image forming apparatuses, based on a plurality of the measured glossiness.