An inverter controller selectively drives inverter switching elements with a preheat frequency (f1), a start frequency (f2), and an ON frequency (f3) which are different from one another, so as to obtain a preheat mode, a start mode, and an ON mode. There are provided reset means for performing start mode operation when voltage supplied to the inverter is lowered than a first threshold value, andinverter stop means for stopping the inverter when an error of the electric discharge lamp is detected. A timer provided for generating a signal to decide start of the preheat mode, the start mode, or the ON mode generates a reset signal inhibit signal for inhibiting operation of the reset means and an inverter stop inhibit signal for inhibiting operation of the inverter stop means. An inverter controller has frequency sweep means for gradually changing the switching frequency from the start frequency to the ON frequency. The timer inhibits operation of the reset means only during a period from the moment when the preheat frequency is selected to the moment when the switching frequency becomes ON frequency. The timer inhibits operation of the inverter stop means only during a period from the moment when the preheat frequency is selected to the moment when the switching frequency begins to change from the start frequency to the ON frequency.