Abstract: An image-forming machine has a developing device that includes a developing agent container for containing a developing agent with toner and carrier particles, and a developing agent application device for applying the developing agent onto an image carrier device. The developing device also includes a toner replenishing device for replenishing the developing agent container with toner, a toner concentration detection device for detecting toner concentration of the developing agent, and a toner replenish control device. The toner replenish control device controls the toner replenishing device by comparing a value detected by the toner concentration detection device with a threshold value.

Abstract: An image formation apparatus for forming a full color image in the four colors of cyan, magenta, yellow and black has a optical ATDC sensor attached in a cyan developing device to control the toner density under optical ATDC. A black developing device has the toner density controlled by AIDC. A standard develop efficiency .eta.STD of the cyan developing device obtained as a result of the AIDC is compared with a develop efficiency .eta. of black toner. The toner supply amount of the black developing device is determined according to the comparison result to supply the toner. Thus, an image formation apparatus is provided that can detect at favorable accuracy the toner density of a developing device incorporating a developer of which the toner density cannot be detected using optical ATDC to supply a required amount of toner.