A light source device is provided. The light source device includes: a ferrule which holds an optical fiber; a light-emitting element which emits laser light; and a casing which houses the ferrule and the light-emitting element such that the laser light enters the optical fiber. The casing includes a through hole from a first end face to a second end face. The through hole includes a diameter which decreases in a stepwise manner from the first end face toward the second end face. The ferrule is fitted to a small bore portion of the through hole, which is smallest in diameter. The light-emitting element is fitted to a large bore portion of the through hole, which is larger in diameter than the small bore portion.