An image forming apparatus includes a main assembly; a cartridge for forming a toner image on a sheet material; a fixing unit 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 the main assembly and are disposed side by side. When the fixing unit and the cartridge are mounted or demounted, 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 a 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 passing through the opening.