In an image-forming apparatus, in which an electrostatic latent image on a photoconductive drum is developed by a toner accommodated in a container, a periodical exchange of carrier in a container is necessary. A method thereof includes steps of fitting a cartridge to the container, discharging all of an old mixture of toner and carrier from the container to the lower chamber, transferring fresh carrier from the carrier compartment into the container, and further, transferring fresh toner from the toner compartment into the container until a predetermined initial color density of the mixture is obtained.