A color image forming apparatus for forming a color image by transferring images of different colors on a recording medium in a multi-transfer mode. The color image forming apparatus includes a plurality of image forming units for successively forming color images of different colors according to image information, a transfer/transport unit for transporting different color images formed by the image forming units in a state that the color images are successively transferred thereon, a detecting unit for detecting patterns used for detecting an out-of-registration of the color images formed on the transfer/transport unit by the image forming units, and a correcting unit for correcting positions of the color images to be transferred on the transfer/transport unit according to data signal output from the detecting unit. In the apparatus, the transfer/transport unit includes a belt having a light transmission characteristic, the detecting unit includes a light source and a photosensing element which are opposed with respect to the belt. The light source includes a light emitting diode, and the photosensing element includes a photosensing element for detecting the patterns used for detecting the out-of-registration of the color images that are formed on the transfer/transport unit, through a lens-array imaging element.