An OADM (100) in a wavelength multiplexing transmission system has a wavelength selection switch (120) which selects the specified wavelength from the multiplexed optical signals multiplexed with phase-modulated signal and intensity-modulated signal to output the selected wavelength to the specified output port. The wavelength selection switch (120) has a delay different for each wavelength of the multiplexed optical signals (for example, the wavelength selection switch (120) includes a mirror array, wherein an optical path from a surface of a mirror arranged on the mirror array to a diffraction grating is different between the adjacent mirrors).
