PURPOSE: To downsize a circuit and cut down the cost and raise reliability and elongate the life by suppressing the change of electric properties by the dispersion of parts used for a lighting circuit, and setting a lamp current and a preheating current to optimum values with little dispersion, and lessening the margin of used parts.

CONSTITUTION: This device is made in such structure that an inverter circuit 9 on lamp side and a preheating circuit 12 are turned on or turned off alternately, and this is provided with circuit means 14 and 15 which detect the lamp current of the inverter circuit 9 and the preheating current of the preheating current 12, respectively, and selectively feed the detected voltage back to a constant voltage circuit 4. The lamp current and the preheated current are controlled to be optimum values by feeding back the detection signal by the lamp current during the on period of the inverter circuit 9 and the detection signal by the preheating current during the on period of the preheating current 12 to the said constant voltage circuit 4 so as to control the output voltage.