The object of the present invention is to provide an image fixing apparatus which enables achieving saving of energy in fixing by controlling the temperature of a heater to fix a toner image on a recording medium, an image forming apparatus having the image fixing apparatus therein, and an image forming process using the same. To achieve the object, there is provided an image fixing apparatus which includes a heater having a heat source to generate heat; a fixing belt which moves in a direction to which a recording medium is transported in a state where the belt comes into contact with the heater; and a pressing portion which presses the transported recording medium with a toner image carried thereon on an area of the fixing belt which comes into contact with the heater, and is characterized in that the contact surface of the heater which comes into contact with the fixing belt has a smoothly formed convex toward the side which comes into contact with the fixing belt with a curvature radius of 200 mm to 100,000 mm, and an amount of contact time between the fixing belt and the recording medium is 20 msec. or more.