PURPOSE: To surely operate a common current detecting device and overcurrent break ing device to perform protection even when a pre-heating device and a fallast corre sponding to any one of plural discharge lamps become abnormal, and further earthing happens.

CONSTITUTION: Plural hot cathode type discharge lamps 2 and 3 are provided between outputs ends of a power source 1, fallasts 6 and 7 provided in correspondence with these discharge lamps respectively, and also pre-heating devices 4 and 5 are provided between non-power-source side electrodes of the respective discharge lamps. Moreover, this device is provided with a current detecting device 6 for breaking a current to the respective discharge lamps when the current in the common current path of the respective discharge lamps exceeds a given value, and an overcurrent breaking device 18 for breaking a current to the respective discharge lamps when the current value in the common current path of the respective discharge lamps becomes a value higher than a given value. This appropriaytely operates the above devices 6 and 18 at the time of the occurences of the abnormality of the pre-heating device and the fallast and earthing and so on respectively to prevent trouble due to the continuation of an abnormal condition.