A light source device includes: a laser diode configured to emit laser light; a substrate directly or indirectly supporting the laser diode; and a cap secured to the substrate and covering the laser diode, the cap including: a first portion configured to transmit the laser light that is emitted from the laser diode, and a second portion that is bonded to the first portion. The second portion includes: a pair of lateral wall portions that are located at lateral sides of the laser diode, the pair of lateral wall portions being bonded to the first portion, a cover portion that is located above the laser diode and connects the pair of lateral wall portions together, and a rear wall portion that faces the first portion with the laser diode disposed between the first portion and the rear wall portion of the second portion.