To prevent lamp extinction during dead time of an inverter circuit of a full bridge system, to reduce delays of the rising time and the falling time of rectangular waves which have been output by the inverter circuit and to prevent formation of a phenomenon of momentary darkening of the radiant light, the discharge lamp is operated with an alternating current with rectangular waves which is produced by an inverter circuit of a full bridge system. When the inverter circuit of the full bridge system is driven a dead time is taken in which all switching devices are turned off. In the rear stage of the inverter circuit of the full bridge system, there is an inductance. The value of the inductance is fixed such that LL≧VL/IL·Td (VL being the luminous voltage of the discharge lamp, IL being the current and Td the dead time). Current can flow in the discharge lamp during the dead time by the energy stored in this inductance.