In the present invention, the safeguarding of the circuit and the step-up transformer can be devised without using the special high-cost components. Voltage supplied by the primary coil (21) of a step-up transformer (2) is switched by a transistor (TR) and the secondary coil (22) of the step-up transformer causes the generation of secondary voltage. In addition to applying said secondary voltage to a cold cathode tube (1), the current flowing through the cold cathode tube (1) is detected, and the transistor (TR) switching function is controlled based on the detection results. To detect the voltage generated by the secondary coil of the aforementioned step-up transformer, a voltage detection coil wrapped around the aforementioned step-up transformer (2) is provided. Abnormalities in the voltage supplied to aforementioned cold cathode tube (1) are detected based on voltage detected by this voltage detection coil (23). Switching by the aforementioned transistor (TR) is stopped when an abnormality is detected.