The invention provides a light source device, a lighting device, a vehicular headlight and a vehicle, wherein the headlight system (100) includes: a semiconductor laser element (11) that emits laser light (L1); a light-emitting section (16) that upon receipt of the laser light (L1) emitted from the semiconductor laser element (11), emits illumination light (L2) which is both the laser light (L1) and fluorescence obtained by wavelength conversion of a portion of the laser light (L1); and a diffusion plate (18) that mixes the laser light (L1) and fluorescence which are contained in the illumination light (L2) emitted by the light-emitting section (16).