The invention reduces loss produced when a power supply device supplies power to a fixing unit of a copier, thereby contributing to a reduction in size, a saving in terms of energy and lower cost. To achieve this, it is arranged so that it is possible to vary the capacitance of a resonant capacitor C200 that constructs a resonant circuit together with a coil 71 through which a high-frequency current flows in order to produce an induced electromotive force in an electrically conductive heating element. In an interval in which a tail current flows after closure of an IGBT 401 that switches the coil current, the capacitance of the resonant capacitor C200 of a switching element SW402 is increased, thereby suppressing a rise in voltage Vce of the IGBT 40 over a fixed interval of time. As a result, loss ascribable to the tail current is reduced.