To provide a method of securing safety of a device by protecting a fluorescent tube by stopping the operation of an inverter when any of a filament of the fluorescent tube and four wires applying tube voltage is broken, preventing the occurrence of a fire or the like caused by arc generation, and detecting an abnormal phenomenon occurring at the end of life of the fluorescent tube.

In order to detect disconnection of a preheating circuit on a high-voltage side of a fluorescent lamp (hot-cathode discharge tube), it is necessary to alternatingly separate a filament winding from a tube voltage application circuit; DC power is generated by using the filament winding, and a closed circuit is formed with a photocoupler PC and a filament of a fluorescent tube to operate a control circuit located on a low voltage side by detecting the disconnection by its opening/closing; and when the disconnection occurs, the operation of the inverter is stopped. The low voltage side can be simply detected.