Images based on each of a plurality of color components are formed; the formed images are transferred on each transfer medium to form an image quality determining image; the image quality of the image quality determining image is determined on the basis of the detected density thereof, an adjustment image is formed by overlaying and transferring an image of other color component to be adjusted to the reference image of the reference color image out of the plurality of color components, on the transfer medium; the density of the adjustment image is detected; and the image forming position of the other color component to be adjusted is adjusted on the basis of the detected density. At the time of executing the color matching adjustment process for detecting the overlaid state of color component images, it is determined whether a specified time has passed or not since the previous adjustment process, a specified number of images are formed or not, environments are changed or not, and the power source is turned on or off or not. When it is predicted that the quality of the formed image is out of an appropriate range, a quality checking image is formed, its image quality is checked, and the color matching adjustment process is executed only when it is within an appropriate range.