A discharge lamp energizing power supply device has a full-wave rectifier circuit for rectifying an AC voltage obtained from a commercial AC power supply system into a full-wave rectified waveform, a booster circuit for boosting the voltage of the full-wave rectified waveform, a boosted-voltage changing circuit for changing the boosted voltage output from the booster circuit, a voltage lowering circuit for lowering an output voltage from the boosted-voltage changing circuit and outputting an activating output voltage for activating a discharge lamp and an energizing output voltage for keeping the discharge lamp energized, and a control device for controlling a boosted voltage in the boosted-voltage changing circuit within a predetermined range based on the voltage of the full-wave rectified waveform when the voltage lowering circuit outputs the energizing output voltage.