An image forming apparatus includes a housing in which an image forming unit is to be mounted, a support that supports an image reading device in a state where the image reading device is separated from the housing, and a guiding portion in which a guide part, which is formed on a to-be-mounted surface of an add-on device in such a manner as to project, is to be inserted such that the guide part is guided by the guiding portion. Engagement holes with which engagement portions, which are formed on the to-be-mounted surface of the add-on device in such a manner as to project, are to be engaged such that the engagement portions are positioned by the engagement holes are formed in a mounting surface of the housing on which the add-on device is to be mounted.