Abstract: A discharge lamp lighting circuit 1 has a DC power supply circuit 3 for outputting a DC voltage, a DC-AC conversion circuit 4 for converting the output voltage of the DC power supply circuit into an AC voltage and then supplying the AC voltage to a discharge lamp 6, and a current detection circuit 8 for detecting an electric current flowing into the discharge lamp. Just after the discharge lamp 6 is lighted, the output frequency of the DC-AC conversion circuit 4 is temporarily changed for defining the lighting frequency of the discharge lamp as a low frequency, to perform DC lighting over a predetermined time. A control circuit 7 determines the duration of the DC lighting by the time it takes for the product of the time and the value of current flowing into the discharge lamp 6 to equal a predetermined value.

Abstract: A vehicular headlamp including a discharge bulb which allows mounting of a lighting circuit case to a lamp body using a simple and inexpensive structure and which allows for easy removal of the lighting circuit case therefrom. Because it is disposed within the lamp body, the lighting circuit case need not have a full waterproof structure, and thus it can be formed as a simple, light-weight and inexpensive structure. The lamp body is composed of a main body having an opening and a cover for closing the opening. A mounting portion for the lighting circuit case is formed at a position closely adjacent the opening of the main body of the lamp body. As a result, the lighting circuit case can be easily removed through the opening.