There is provided an image forming apparatus which adopts a distributed control system and increases the error detection accuracy of each control unit. To accomplish this, the image forming apparatus includes a master control unit that controls the overall image forming apparatus, a plurality of sub-master control units that control a plurality of functions for performing image formation, and a plurality of slave control units that control loads for implementing a plurality of functions. The master control unit determines a diagnosis path for performing a diagnosis process for an error using the signal lines and a connection bridge connected to the respective control units. The master control unit performs the diagnosis process for an error in accordance with the determined diagnosis path.