In a lighting circuit for a discharge lamp, the discharge lamp is kept lit without fail when a DC input voltage becomes low, while avoiding a complicated circuit configuration and control method. A DC-AC converter circuit receives a DC input voltage and converts it to an AC voltage and boosts the AC voltage. The DC-AC converter circuit is controlled by a control means having a circuit for detecting the DC input voltage "+B" to perform a lighting control of the discharge lamp. The lighting circuit comprises a transformer for AC conversion, switching elements, and resonance capacitor. The switching elements are driven to cause a series resonance of the resonance capacitor and an inductance component of the transformer for AC conversion or an inductance element. Upon detection of a reduction in the DC input voltage, the switching element driving frequency is shifted to a frequency lower than a frequency range when the discharge light is lit to increase the output voltage of the DC-AC converter circuit, thus maintaining the lighting state of the discharge lamp.