A circuit conductor 25 is provided on a base 19. A semiconductor laser 5 is connected to the circuit conductor 25. Cutout parts on which the circuit conductor 25 is not formed are provided at, e.g., the vicinity of the four corners of the base 19, and a hole 20 is provided at each of the said portions. The holes 20 penetrate the base 19. Fixing members 21 are inserted through the holes 20. The fixing members 21 are, e.g., male threads. Since the head part of the fixing members 21 is located in the cutout part, the fixing members 21 and the circuit conductor 25 are not in contact with each other. A platform 17 has holes formed at portions corresponding to the holes 20 in the optical unit 23 and female threads formed on the inner surface. The fixing members 21 and the platform 17 are therefore joined. As a result, the optical unit 23 is fixed to the platform 17.