Image forming apparatus, united interface device, and method for using option unit

The image forming apparatus is disclosed. This device, of at least one type of the option units are integrated interface that can be connected individually, connected to the power source, integrated interface option unit, a plurality of switches provided on the power transmission path between the power source, and, the optional unit a plurality of switches such that transmission of the information and the type size of the option unit power of the corresponding to the using the previously stored ID information including the power size information allowed, check the type of the optional units, and determine the type in accordance with to the type of a control unit for controlling on / off. Thus, optional unit can be prevented from damage or the like of the image forming apparatus which may occur, by being incorrectly connected to the image forming apparatus. Optional unit, the main unit, the image forming apparatus, ID information