A discharge lamp driving circuit includes an inverter, a ballast capacitor, a discharge lamp, and a lamp current detecting circuit. The inverter converts a DC voltage into an AC voltage with high frequency to output the AC voltage to an output port based on a pulse width modulation control signal. The lamp current detecting circuit outputs a first voltage signal and a second voltage signal according to a voltage across the ballast capacitor to generate a lamp current sensing voltage that is proportional to a lamp current flowing through the discharge lamp. The pulse width modulation control signal has a width varying with amplitude of the lamp current so that the lamp current may be accurately detected.