In an optical add/drop multiplexer dropping and inserting a light of a specific wavelength for a WDM signal light and an optical network system utilizing the same, a wavelength selective switch selects a part of WDM signal lights inputted as lights of drop wavelengths and outputting other lights, and a reject/add filter terminates lights of same wavelengths as add wavelengths among output lights from the wavelength selective switch and multiplexes other lights than the terminated lights and lights of the add wavelengths to be outputted. At this time, the drop wavelengths are made to include all of the add wavelengths.