A color laser printer of the invention identifies each object image to be printed as a color image or a monochromatic image. When the object image to be printed is identified as a color image, the control procedure of the invention rotates a developer unit with multiple color toner cartridges to sequentially move positions of the respective color toner cartridges to a development position and forms respective color toner images on a photoreceptor. When the object image to be printed is identified as a monochromatic image, on the other hand, the control procedure fixes a specific toner cartridge filled with black toner to the development position and forms a black toner image on the photoreceptor. A counter counts up consecutive monochromatic printing operations. After a preset number of the consecutive monochromatic printing operations in the fixed state of the developer unit, the developer unit is rotated to return and fix the specific toner cartridge filled with black toner to the development position for monochromatic printing.