Color image forming apparatus includes a belt-shaped image carrier stretched and conveyed around rotating rollers on which image exposure is conducted by a plurality of exposure units; and a plurality of step-formed supporting surfaces below the lower surface disposed of the image carrier. The stepped formation of the exposure unit supporting surfaces avoids interference between optical paths of laser beams for image exposure, and facilitates compactness of the apparatus. The belt-shaped image carrier and the supporting surfaces are provided at a relative angle of 5-30 degrees. The color image forming apparatus is also capable of detecting a specific pattern outputted from the exposure units; calculating the differences of positions of the specific pattern as formed by the different exposure units; and controlling the positioning of the specific pattern as outputted by the exposure units, so as to increase the accuracy of the image forming apparatus.