An image forming apparatus has a functional redundancy system by which an inherent function normally performed by a certain portion is performed as a potential function by another portion, when such certain portion no longer can be perform its inherent function. In a first operative sequence, portions in the image forming apparatus perform their respective inherent functions. However, where a function of a certain portion in the apparatus is lost, for example, where a fault occurs and a particular element has failed, operation of other portions in the apparatus are controlled according to a second sequence. In the second sequence, the inherent function of the portion which has failed is replaced by the potential function of another portion in the apparatus.