An induction heating circuit for a fixing device of an image forming apparatus, the induction heating circuit comprising: an electrically conductive heating element for generating heat using an induction heating method, the electrically conductive heating element being made of a magnetic material which has a characteristic that the magnetic material loses magnetism at a temperature equal to or higher than a Curie temperature; an induction coil configured to generate a magnetic field to cause the electrically conductive heating element to generate heat; a capacitor connected to the induction coil; driving signal generation means (20, 112) configured to determine a frequency of a driving signal (121, 122) which becomes equal to or higher than a set minimum frequency (Fmin1, Fmin2, Fmin3) for providing electric power to the induction coil and to generate the driving signal (121, 122); temperature detection means configured to detect a temperature of the electrically conductive heating element; and control means configured to adjust the minimum frequency (Fmin1,Fmin2, Fmin3) according to the temperature detected by the temperature detection means and to cause the driving signal generation means (20, 112) to determine the frequency of the driving signal (121, 122) so that the frequency of the driving signal (121, 122) becomes equal to or higher than the minimum frequency (Fmin1, Fmin2, Fmin3) which is equal to or greater than a resonant frequency (fpy1, fpy2) of the induction heating circuit determined based on inductance of the induction coil, inductance of the electrically conductive heating element and capacitance of the capacitor, such that the driving frequency of the driving signal (121, 122) is always equal to or higher than the resonant frequency (fpy1, fpy2). an electrically conductive heating element for generating heat using an induction heating method, the electrically conductive heating element being made of a magnetic material which has a characteristic that the magnetic material loses magnetism at a temperature equal to or higher than a Curie temperature; an induction coil configured to generate a magnetic field to cause the electrically conductive heating element to generate heat; a capacitor connected to the induction coil; driving signal generation means (20, 112) configured to determine a frequency of a driving signal (121, 122) which becomes equal to or higher than a set minimum frequency (Fmin1, Fmin2, Fmin3) for providing electric power to the induction coil and to generate the driving signal (121, 122); temperature detection means configured to detect a temperature of the electrically conductive heating element; and control means configured to adjust the minimum frequency (Fmin1,Fmin2, Fmin3) according to the temperature detected by the temperature detection means and to cause the driving signal generation means (20, 112) to determine the frequency of the driving signal (121, 122) so that the frequency of the driving signal (121, 122) becomes equal to or higher than the minimum frequency (Fmin1, Fmin2, Fmin3) which is equal to or greater than a resonant frequency (fpy1, fpy2) of the induction heating circuit determined based on inductance of the induction coil, inductance of the electrically conductive heating element and capacitance of the capacitor, such that the driving frequency of the driving signal (121, 122) is always equal to or higher than the resonant frequency (fpy1, fpy2).