To provide a fixing device in which the heat of an induction heating body is not conducted outside the area where a printing medium is passed.

The fixing device includes: a magnetic flux generating part which generates a magnetic flux; an induction heating body which is disposed within the magnetic flux, heats by electromagnetic induction and heats a toner image on a printing medium; and a changing part which selectively changes the length of the heating portion of the induction heating body according to the size of the printing medium. The dimension of the induction heating body is selectively changed such that the heating portion of the induction heating body extends over at least the width of the image area of the printing medium where the toner image is formed. The induction heating body includes two or more induction heating body elements of different lengths which extend laterally relative to the running direction of the printing medium, and the length of the heating body is selectively changed in accordance with the width of the printing medium. A current supplied opposite the heating part differs depending on whether the induction heating body is within the magnetic flux or not. Based on the difference, the induction heating body is positioned.