PURPOSE: To improve electric discharge lamp control efficiency by supplying electric power to a discharge tube while changing output voltage of a switching regulator according to discharge tube voltage displacing according to a temperature change in the discharge tube.

CONSTITUTION: Voltage VL of a discharge tube L is detected by a detecting part 13, and a pulse width control part 14 changes pulse width according to the VL, and controls a switching period of a switching regulator 15, and outputs output voltage VS. Here, when VS is set on a prescribed level higher than the VL, stable voltage can be supplied to the discharge tube L while keeping always a prescribed preset level in response to the voltage of the discharge tube. Thereby, an electric discharge lamp can be controlled efficiently.