A light source device includes: a first laser light source which emits first laser light; and a diffusion element which is provided on an optical path of the first laser light and diffuses the light, wherein the diffusion element includes: a first lens array in which a plurality of first lens elements that divide the first laser light into a plurality of light fluxes are arranged; and a second lens array in which a plurality of second lens elements that emit the light fluxes incident via the plurality of first lens elements toward an image formation plane are arranged, wherein the first lens array is disposed on an incidence surface of the diffusion element, wherein the second lens array is disposed on an emission surface of the diffusion element, and wherein at least two second lens elements among the plurality of second lens elements form light source images having different sizes in different regions on the image formation plane.