In a color laser printer, the amount of reversely transferred developing agent on the surface of a developing roller is estimated based on the operating time of the color laser printer, the surface area of an image, and the charging amount of developing agent. When the estimated amount of reversely transferred developing agent reaches a predetermined value, a solid image is formed on a photoconductor drum. By transferring the solid image onto recording paper, a conveyer belt, or an intermediate transfer belt, the reversely transferred developing agent is removed. Alternatively, by rotating the developing roller in a state in which the developing roller is separated from the photoconductor drum, the reversely transferred developing agent on the surface of the developing roller is collected into a developing agent hopper.