1. An image forming apparatus for forming an image with color toners including black toner, comprising: an image forming device that forms, on an image forming medium, patch images for detecting density of the image and trigger images for generating timing to read the patch images, by developing electrostatic latent images formed on an image bearing member with the color toners; a detecting device that detects the trigger images formed by said image forming device and reads the patch images in response to the detecting result of the trigger images; and a controlling device that controls a density of the image based on the patch images read by said detecting device; wherein the patch images are comprised of a plurality of single-colored patch images respectively formed with different color toners other than the black toner and the trigger images are formed respectively corresponding to the plurality of single-colored patch images, and wherein said detecting device reads, when detecting one of the trigger images, one of the plurality of single-colored patch images, corresponding to the detected one of the trigger images, so as to detect the density thereof.