To provide a fixing unit and an image forming apparatus where the radiation of heat from a casing to the outside of the casing is restrained to the utmost, temperature rise at a part positioned on the upside of the casing is restrained and also temperature fall inside the casing is restrained so as to make power consumption low.

The fixing unit 8 is constituted so that recording paper P carried nearly in a vertical direction from the downside to the upside of the casing 80 passes between a fixing roller 81 and a pressure roller 83 so that an image may be fixed, and then it is ejected through an upper aperture 93 formed on the casing. Then, an opening/closing means 95 opened when the recording paper P passes through the aperture 93 and closed when it does not pass through the aperture 93 is disposed on the unit 8.