A discharge lamp lighting apparatus is provided which includes: a high-voltage transformer; a switching circuit to drive the primary side of the transformer; a frequency changing means to change an operating frequency of the switching circuit according to lighting conditions of a discharge lamp; and first and second resonant circuits formed respectively at the primary and secondary sides of the transformer and having respective specific resonant frequencies different from each other. The switching circuit, before the discharge lamp is turned on, operates at a frequency ranging in proximity of the series resonant frequency of the secondary resonant circuit, and after the discharge lamp is turned on, operates at a frequency ranging in proximity of a frequency at which a voltage-current phase difference at the primary side of the transformer is minimized. The capacitance component of the second resonant circuit is constituted by a parasitic capacitance only.