In a WDM transmission system, a wavelength selecting switch demultiplexes a first WDM light containing a plurality of wavelength components into a plurality of single wavelength lights each containing corresponding one of the wavelength components, effects multiplexing on the plurality of single wavelength lights to form a second WDM light, and effects demultiplexing on the outputted second WDM light. A wavelength demultiplexing unit demultiplexes the demultiplexed second WDM light into a plurality of wavelength demultiplexed lights for every wavelength component so that physical quantity monitoring can be made on each wavelength demultiplexed light deriving from the demultiplexing. In this way, transmission channel switching can be independently effected on the wavelength light having a single component contained in the WDM light, wavelength demultiplexing elements provided in a spectral optical system can be shared to reduce the number of elements, and the wavelength selection switch can be assembled with ease.