Conventionally, an image forming apparatus comprises: an image carrier on the surface of which an electrostatic latent image is formed and a toner is adhered to the electrostatic latent image by a developing unit; and a transfer unit that transfers the toner adhering to the image carrier to a recording medium.  With this kind of image forming apparatus, when the electrostatic latent image is formed on the surface of the image carrier, the toner is adhered to the electrostatic latent image by the developing unit.  When the toner adhering to the image carrier is transferred to the recording medium by the transfer unit, a toner image can be formed on the recording medium in accordance with the electrostatic latent image. 
Also, there is an image forming apparatus of this kind equipped with four image carriers in a main body of the image forming apparatus.  The colors of toners adhered by developing units to the image carriers are yellow, magenta, cyan and black.  The four color toners are transferred to be superimposed on a recording medium to form a color image.  With this kind of image forming apparatus, each of the image carriers is configured to be individually attachable and removable, and the image carriers are appropriately replaced in accordance with a lifespan. 
In the image forming apparatus equipped with the plurality of image carriers, when a user attaches the image carriers in a wrong arrangement order, an effective image formation may become impossible.  According to an image forming apparatus disclosed in JP-A-2001-343883, when the arrangement order of the image carriers is wrong, a user is informed of the abnormality.