To provide a color image forming device of an electrophotographic system, capable of executing feedback control of color image density in an arbitrary printing image, without requiring printing a color patch.

The device can be constituted of images to be measured in a minute image by calculating an amount of reduction of paper from before and after fixation, based on the relation of the image position after fixation and the write-in position prior to fixation, calculating the position of the image to be measured after fixing, reading the concentration, and executing a process control to it.