When the maintenance mode is not set by the mode setter 102, the controller 100 may activate the light emitters T1 to T4 and the light receivers R1 to RA, at a predetermined time point for checking (e.g., every hour, or every day). In this case, when the identifier 103 is unable to identify the color of received light, or decides that all of the light intensity signals are of a level not higher than the predetermined threshold, with respect to at least one of the light receivers R1 to RA, the controller 100 causes the display device 473 to display a message to the effect that there is a malfunction in the light receiver R about which the color of received light is unable to be identified, in the light receiver R in which all the light intensity signals are of a level not higher than the predetermined threshold, or in the corresponding light emitter T.
Hereunder, an example of the operation performed by the control device 10 of the image forming apparatus 1 according to the first embodiment will be described, with reference to