PURPOSE: To output a multilevel image with stable density by deciding the operating condition of an image forming means based on the measured density of a sample image and detected toner concentration.

CONSTITUTION: Respective color toner images with fixed density successively formed on a specified image forming condition are successively read by using a black toner concentration detecting sensor 600, and the read data is fed back to the image forming condition of each color. As to color toner such as yellow, magenta and cyan; reflected light quantity becomes larger than in the case of a photoreceptor drum 19 being a simple substance and output from the sensor 600 becomes larger as an area covering rate becomes larger and the density of an output image becomes higher. Meanwhile, as to black toner, the reflected light quantity becomes smaller than in the case of the photoreceptor drum 19 being the simple substance and the output from the sensor 600 becomes smaller as the area covering rate becomes larger and the density of the output image becomes higher. By using such relation, the density of the output image is accurately obtained based on the output from the sensor even in the case of the toner having different reflection characteristic.