To mitigate the formation of an excessive transient phenomenon in a deformation half-bridge type discharge lamp lighting device and to prevent the breakage of elements.

This lighting device is provided with a pair of switching elements 2, 3 connected in forward series, flywheel diodes 4, 5 connected in reverse parallel to the switching elements 2, 3, a DC power supply 1 connected astride the switching elements 2 3, and lighting circuits 6, 7 including a ballast inductor 6 and a discharge lamp 7 in series. One end of the lighting circuits 6, 7 is connected to the connection center point of the switching elements 2, 3, and the other end of the lighting circuits 6, 7 is connected to one end of the DC power supply 1 via an auxiliary capacitor 9. This deformation half- bridge type discharge lamp lighting device is provided with an auxiliary diode 10 connecting the other end of the lighting circuits 6, 7 to one optional end of the DC power supply 1 and having the reverse polarity to the voltage of the DC power supply 1.