A system for safeguarding 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. To detect the voltage generated by the secondary coil of the step-up transformer, a voltage detection coil wrapped around the step-up transformer (2) is provided. Abnormalities in the voltage supplied to a cold cathode tube (1) are detected based on voltage detected by this voltage detection coil (23). Switching by the transistor (TR) is stopped when an abnormality is detected.