An image forming apparatus includes: an image forming portion; an image forming portion supporting frame having opposing first and second side surfaces; first and second frame supporting portions provided, in contact to a disposition surface of the apparatus, close to the first side surface; a third frame supporting portion provided, in contact to the disposition surface, close to the second side surface; and an auxiliary leg adjustable to a height where it does not contact the disposition surface and a height where it contacts the disposition surface. The auxiliary leg is provided in a side which is away from the third supporting portion in a direction of change of the center of gravity by mounting of the optional unit to the image forming apparatus.