An image fixing apparatus includes a heater; a film movable in contact with the heater at one side thereof and in contact with a recording material at the other side thereof, wherein a visualized image on the recording material is heated and fixed by heat from the heater through the film; the heater including a heat generating layer extending in a direction crossing a movement direction of the film, for producing the heat by the application of a voltage between its longitudinal opposite ends; an electric path branched out of an image fixing part of the heat generating layer; and a selector for selecting the electric path in accordance with a size of the recording material.