A CPU 113 executes a control program stored in a memory 116 thereby forming a plurality of patch images with a developing bias increased for each of the patch images; detecting a density of each of the patch images by means of a patch sensor 17; and comparing the image densities to determine whether the image density is saturated or not. Then, an image forming condition in which an image density is substantially saturated is determined and is stored in the memory 116. When a print command signal is inputted from an external device via a main controller 100, a printing operation is performed under the image forming condition stored in the memory 116.