In image formation of a black-line image, the image is developed with black toner, and in addition parts of the black-line image corresponding to an intersection between at least two black lines and a middle portion of a black line are developed with toner of at least one of cyan, yellow, and magenta colors overlaid. This makes it possible to prevent central voids in a black-line image. In a black-line image transferred onto a transfer medium by this full-color image forming apparatus, not only are central voids eliminated, but degradation of image quality is also prevented.