An image forming apparatus includes an unfixed-image forming unit for forming an unfixed image, and a fixing unit. The fixing unit includes a heating member for heating the unfixed image, and a heat insulating member having an interval with the heating member and disposed so as to cover the heating member along the heating member in a direction perpendicular to a conveying direction of a recording material, for insulating heat from the heating member. The fixing unit is detachably mountable in the image forming apparatus, and fixes the unfixed image on the recording material formed by the unfixed-image forming unit. The apparatus also includes an exhaust fan disposed near the fixing unit, for exhausting air from within the image forming apparatus. The fixing unit has a ventilation channel passing through the fixing unit outside of the heat insulating material.