The oscillating switching element of step-up transformer comprises a dedicated high-frequency FET and is placed between center tap of the primary coil and the ground of the primary circuit. Terminal of the primary coil on the side of the ground is connected to the connection point between xenon tube and the light emission controlling switching element, i.e., an IGBT, via diode. Charge voltage of main capacitor is detected by voltage detection circuit, and when charge voltage is below threshold voltage, controller drives the IGBT at a low frequency, while when charge voltage has exceeded threshold voltage, it drives the FET at a high frequency to charge main capacitor. The charging of main capacitor is made faster and more efficient by changing the switching frequency and by switching the switching element between the IGBT and the FET as charge voltage increases.