To solve problems that more precise control for the oscillation wavelength of a semiconductor laser diode is required for the wavelength division multiplexing transmission with large capacity but that the structure of the device is complicated and its cost increases when the oscillation wavelength is controlled with high accuracy.

Each diode in a semiconductor laser diode group emits the fundamental wave light having the controlled wavelength as twice as the wavelength of light to be transmitted. The output light enters the entrance 2 for light and converted into light having 1/2 wavelength by the group 3 of wavelength conversion elements. The group 3 of wavelength conversion elements emits the excess fundamental wave light and converted light with 1/2 wavelength as that of the fundamental wave light. The output light passes the group A of fundamental wave light removing part such as an optical filter which transmits the converted light and removes the excess fundamental wave light and passes light output part 5 and optical multiplexing part 6 and guided to a transmission medium 7.