PURPOSE: To prevent stress or electric power loss from increasing since both end voltages of a switching element are increased as a light dimming degree becomes small when light dimming control is carried out on an electric discharge lamp in a semiconductor ballast system electric discharge lamp lighting device formed by using the switching element as ballast while impressing direct current voltage obtained by rectifying and smoothing alternating current electric power supply on the electric discharge lamp after being converted into high frequency electric power supply by means of the switching element.

CONSTITUTION: A control circuit S0 is provided to change a level of direct current voltage Vdc in the direction becoming approximately equal to lamp voltage Vh changing according to a light dimming degree. Thereby, since ON- stage voltage Vce of a switching element Q1 can be reduced when lighting output of an electric discharge lamp H is large, an effect such as stress or electric power loss can be reduced in the switching element Q1, can be obtained.