An image forming apparatus includes an image reading portion; an image forming portion; a first casing; a second casing including a space which continuously opens to two directions perpendicular to not only the vertical direction but also each other and including a mounting portion where the first casing is mountable at a position where the space does not continuously open to the two directions; one or more supporting portion; and one or more auxiliary supporting portion. A gap is formed between itself and the other one of the first casing and the mounting portion in a state in which no load is externally exerted on the first casing, and wherein the auxiliary supporting portion contacts the other one when a load not less than a predetermined load is externally exerted on an upper portion of the first casing downward at a position deviated from the mounting portion.