A coil for generating a high-frequency magnetic field is provided in a heating roller. The coil is connected to a capacitor, constituting a resonant circuit. The resonant circuit is incorporated in a high-frequency wave generating circuit. A current-detecting circuit detects the current supplied to the high-frequency wave generating circuit. The supply of the current to the high-frequency wave generating circuit is controlled in accordance with the current detected by the current-detecting circuit and the logic level of an operation-on signal.