PURPOSE: To dispense with a DC/DC converter while maintaining high efficiency by arranging a constant voltage circuit to the positive side of a filament and the voltage setting circuit and an FET to the negative side of the filament.

CONSTITUTION: When a constant voltage having 8V from an 8V constant-voltage circuit 6 is always impressed on the positive side of a filament 3 and the negative side of the filament 3 is made to be turned ON/OFF with the same timing as in a conventional one, the same waveforms as in the conventional one are obtained eventually. Specifically, an FET 7 is built-in at the negative side of the filament 3. Then, the FET 7 is a semiconductor element constituting a switching circuit and the semiconductor element in which a drain D and source S are made ON by impressing a constant voltage (a voltage being equal to or higher than 2V) in between a gate G and the source S is selected. Moreover, R1 and R2 sets previously the voltage (1.5v) of a degree with which the FET 7 is not turned ON. Furthermore, differential waveforms due to a capacitance coupling are made by C2 and R2.