A light source device includes: a semiconductor laser configured to emit light; a coupling lens configured to convert the light from the semiconductor laser into light beam; and a holder supporting the coupling lens. The coupling lens has: an optical surface defining an optical axis; and a flange portion protruding outward in a radial direction orthogonal to the optical axis. The flange portion includes: a first flange portion having an arcuate surface centered on the optical axis; and a second flange portion having a flat surface extending along the optical axis. The holder has: a first positioning part opposing the arcuate surface in the radial direction for positioning of the coupling lens; and a first notch through which a part of the flat surface is exposed in the radial direction. At least a part of the first flange portion is bonded to the holder.