1.  Field of the Invention
The present invention relates to a vehicle headlamp for changing over and illuminating a light distribution pattern for low beam (light distribution pattern for passing), a light distribution pattern for high beam (light distribution pattern for cruising), and a light distribution pattern for daytime running light toward a forward direction of a vehicle. 
2.  Description of the Related Art
A vehicle headlamp of this type is conventionally known (Japanese Laid-open Patent Application No.  2007-109493).  Hereinafter, the conventional vehicle headlamp will be described.  The conventional vehicle headlamp is made up of: a first light source unit which forms a light distribution pattern for low beam; and a second light source unit which forms a light distribution pattern for high beam.  The first light source unit is of a projector-type lamp unit, and is provided with a light source, an elliptical (convergent) reflector, a shade, and a projecting lens.  In addition, the second light source unit is a projector-type lamp unit, and is made up of a light source, an elliptical (convergent) reflector, and a projecting lens.  Hereinafter, functions of the conventional vehicle headlamp will be described.  When a light source of the first light source unit is lit, light from the light source is reflected by means of the reflector; a part of the reflected light is cut off by means of the shade; a light distribution pattern having an oblique cutoff line and a horizontal cutoff line, i.e., a light distribution pattern for low beam is formed; and the light distribution pattern for low beam is longitudinally or transversely inverted from the projecting lens, and is illuminated (projected) toward the forward direction of the vehicle.  In addition, when a light source of the second light source unit is lit, light from the light source is reflected by means of the reflector, and the reflected light is longitudinally or transversely inverted from the projecting lens, as a light distribution pattern for high beam, and is illuminated (projected) toward the forward direction of the vehicle. 
Again, the conventional vehicle headlamp is made up of: the first light source unit having the light source, the reflector, the shade, and the projector lens; and the second light source unit having the light source, the reflector, and the projecting lens.  Thus, the conventional vehicle headlamp requires a large number of components and requires the second light source unit for high-beam light distribution pattern, entailing a problem concerning downsizing, weight reduction, power saving, or cost reduction, accordingly.  In addition, in order to obtain a light distribution pattern for daytime running light, the conventional vehicle headlamp requires a third light source unit having a light source, a reflector, a shade, and a projecting lens, in addition to the first and second light source units.  Therefore, in order to obtain the light distribution pattern for daytime running light, the conventional vehicle headlamp further entails problems concerning downsizing, weight reduction, power saving, and cost reduction. 
The present invention has been made in order to solve the above-described problem concerning downsizing, weight reduction, power saving, or cost reduction, which could arise due to a reason that the conventional vehicle headlamp requires the second light source unit for high-beam light distribution pattern.