An image forming apparatus includes a main assembly; a cartridge, detachably mountable to the main assembly, for being operated so as to form a toner image on a sheet material in a state in which the cartridge is mounted in the main assembly; a fixing unit, detachably mountable to the main assembly, for fixing the toner image on the sheet material; and an opening for exposing an inside of the main assembly. The fixing unit and the cartridge are mountable into and demountable from the main assembly in a mounting and demounting direction in which the fixing unit and the cartridge are disposed side by side. When the fixing unit and the cartridge are mounted into and demounted from the main assembly, the fixing unit passes through the opening, and the cartridge passes through a space in which the fixing unit is to be mounted and passes through the opening. When the fixing unit and the cartridge at the time of passing through the opening are projected onto a surface perpendicular to the mounting and demounting direction, a contour of a projected area of the cartridge is at least partly inside a contour of a projected area of the fixing device, and the cartridge passes through the inside of the contour of the projected area of the fixing device when passes through the opening.