A semiconductor light source includes multiple light-emitting elements coupled in series. A driving circuit receives an input voltage, and supplies a driving current to the semiconductor light source. A bypass circuit is coupled to a part of the multiple light-emitting elements. When a low-voltage state occurs, the bypass circuit is set to the enabled state, thereby bypassing the driving current. An auxiliary power supply circuit is coupled in series with the part of the multiple light-emitting elements. When an expected current does not flow through the auxiliary power supply circuit after the bypass circuit is set to the disabled state, a judgement unit judges that an open-circuit fault has occurred in the part of the multiple light-emitting elements.