An image forming apparatus includes a main assembly, an image forming portion for forming a toner image using an electrophotographic process, an air supply opening for fluid communication between an inside and an outside of the main assembly, and an air supply device for supplying ambient air into the main assembly through the air supply opening. In addition, an exhausting device discharges air from inside of the main assembly, and a mounting portion selectively mounts, to the air supply opening, one of a cooling device for taking the ambient air in and for decreasing a temperature of the air taken in, and a cover for partly blocking the air supply opening and forming an air passage portion. When the cooling device is mounted to the mounting portion, the air supply device supplies the cooled air into the main assembly, and when the cover is mounted to the mounting portion, the air supply device supplies the ambient air directly into the main assembly through the air passage portion of the cover. When the cooling device is not mounted to the mounting portion, a control device continues air supply from the air supply opening irrespective of a detected temperature, and when the cooling device is mounted to the mounting portion, the control device controls on and off of the cooling device on the basis of an output of the detecting device, and on and off supply of air from the air supply opening in interrelation with the cooling device.