To provide a wavelength multiplex transmission system which can have its operation wavelength extended, without varying the total output level at the use end of an device in-use and an output level per wavelength in-use.

n/2 λiCW light generation part 1-j generate CW lights having the same wavelength as the wavelength in-use, and output CW lights which have the level twice as high as the input level of a light signal with the wavelength in-use inputted from a λi input terminal. Then n/2 λi switching circuit parts 2-j output one of the CW lights from the λiCW light generation parts 1-j and the operation in-use from the λi input terminal to a wavelength multiplexing part 4. A control part 3 outputs a switching control signal and a CW light output level adjustment control signal to the λi switching circuits 2-j according to the operation state of the wavelength. The wavelength multiplexing part 4 multiplexes the wavelengths of light signals from input terminals λ1, λ3, and the like and the different-wavelength light signals from the λi switching circuits 2-j, and outputs the resulting signal to an output terminal.