To provide a high pressure discharge lamp lighting device which is a lighting device for lighting an HID headlamp used for a vehicle, does not cause a transformer core to be magnetically saturated even when startup lighting, makes a DC-DC converter circuit smaller/lighter, and in turn, can make the lighting device smaller/lighter.

A PWM controller 6 comprises a power controller 31 into which a voltage detecting signal is inputted, and which controls the duty of a signal waveform of a PWM control signal; an error amplifier 32 which compares the output from the power controller 31 and a reference voltage, and outputs an error level signal; an oscillation frequency controlling means 36 which outputs a rectangular wave signal so that the rectangular wave signal is a high frequency during a specified period after the startup lighting of an HID lamp 5, and a low frequency after the specified period; a triangular wave oscillator 34 which converts the rectangular wave signal into a triangular wave signal; and a comparator 35 which outputs the PWM control signal to switching elements 12, where the PWM control signal becomes high level in the period that the rectangular wave signal is larger than the error level signal when the two signals are compared.