Provided is a light source device, including: a base member; a semiconductor laser disposed on the base member; a lateral wall portion formed so as to surround the semiconductor laser; a light-transmissive lid covering a gap surrounded by the base member and the lateral wall portion; and a connection member that airtightly connects an upper surface of the lateral wall portion and a lower surface of the lid over an entire perimeter of the lateral wall portion. The lateral wall portion has a reflecting surface which is an inside surface connected to an upper surface, the reflecting surface being inclined so that light emitted from the semiconductor laser is reflected toward the lid. A dielectric film is continuously formed on the reflecting surface and the upper surface. A height of the connection member is greater than a height of the dielectric film formed on the upper surface.