1. A method for controlling gloss difference in a plurality of image forming apparatuses 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; receiving information indicating one of the plurality of image forming apparatuses displayed on an operating unit, selected according to a manual inputting operation based on the 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 other image forming apparatuses, based on the measured glossiness relating to the selected image forming apparatus.