An image forming apparatus comprises a main body casing; a fixing unit which fixes a developer to a sheet by means of heating and pressurization; a fan which is provided to discharge air inside the main body casing to outside of the main body casing; and a controller. The fixing unit includes a heating unit which heats the sheet; and a temperature sensor which detects a temperature of the heating unit. When the fixing unit is fixing the developer to the sheet, the controller is configured to: determine a heating unit temperature based on the temperature detected by the temperature sensor; stop the fan in a case of determining that the heating unit temperature is not less than a threshold value; and drive the fan in a case of determining that the heating unit temperature is less than the threshold value.