A fixing device of an image forming apparatus of the present invention enables induction heating coils for a 100 V power source to set a ratio of inductance L to load resistance R of a heat roller to L/R <35.times.10.sup.-6 (H/.OMEGA.) and coil impedance Z (.OMEGA.) to Z<10 (.OMEGA.) and supplies a drive current at a high frequency of 40 to 70 kHz. By doing this, an eddy current generated in the heat roller is concentrated upon a metallic conductive layer by the skin effect and the heat generation efficiency of the heat roller is improved.