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