placed on the optical fiber 12 and bonded thereto using an adhesive then the si substrate 13 with the optical fiber 12 is assembled in a case 15 and is bonded to the center of the bottom surface of the case 15 then the optical fiber 12 is guided out through a hole made through the side wall of the case 15 to which a u shaped fiber guide 20 is fixed for protecting the optical fiber 12 the optical fiber 12 is basically covered with a nylon coating 21 protecting itself against an external force and when the optical fiber 12 is mounted on the si substrate 13 the coating 21 is removed to fix the optical fiber 12 with high positioning accuracy and with reliability a plate 16 with good thermal conductivity is mounted on the bottom surface of the case 15 after having been assembled in the case 15 an external electrode terminal 17 and a respective electrode 18 on the si substrate 13 are electrically connected to each other the external electrode terminals 17 are mounted on the side wall of the case as shown in fig1 a such that each external electrode terminal 17 is at the same level from the bottom surface of the case as the top surface of the si substrate 13 finally a resin is potted on the semiconductor laser element 10 and the semiconductor light receiving element 11 to protect them against dust moisture and the like next one preferred embodiment of the inside structure of the case 15 will described with reference to the perspective.