A tandem type color image forming apparatus, which can continuously form color images each containing two or more colors, including: a signal creating section having, for each color, two or more image area setting counters for generating an image valid area signal to set a starting position and a width of an image formation area in a sub-scanning direction; and a control section for executing color image formation control on a predetermined face of paper, based on an image top signal for controlling writing of an image on an image carrier and the image valid area signal for each color, wherein the control section sets each of the outputs of the image area setting counters independently according to a preset image formation mode, and selects and controls one of the image area setting counters for each image formation of each color.