A light source device includes a sub-mount, a semiconductor laser element flip chip mounted on the sub-mount, and a planar lightwave circuit (PLC) which is an optical member having a waveguide disposed on a substrate. The semiconductor laser element and the waveguide are arranged such that a light-emitting point of the semiconductor laser element and a core of the waveguide are substantially aligned. A light-reflecting surface is provided such that light emitted from the semiconductor laser element and propagating along a propagating direction in the planar lightwave circuit is reflected at the light-reflecting surface in a direction substantially normal to the propagating direction. A portion of the substrate of the planar lightwave circuit is removed at least in a predetermined range from an end opposite to a light incident surface of the planar lightwave circuit.