A light source device 1 includes a laser light source 10 and an optical phase modulator 15 or the like. The optical phase modulator 15 inputs coherent light output from the laser light source 10 and transmitted through a beam splitter 14, phase-modulates the light according to the position on a beam cross section of the light, and outputs the phase-modulated light to the beam splitter 14. When (p+1) areas sectioned by p circumferences centered on a predetermined position are set on a beam cross section of light input to the optical phase modulator 15, the more outside each of the (p+1) areas is, the wider the radial width of the area, the amount of phase modulation is constant in each of the (p+1) areas, and the amounts of phase modulation differ by π between two adjacent areas out of the (p+1) areas.