To obtain a wavelength-group wavelength converter which enables wavelength-group exchanging, while keeping the size small and the cost low, without deteriorating the efficiency of exchanges.

AWG 10 divides an input wavelength-group into wavelength units, wavelength converters 21-2n convert the wavelengths of the wavelength units and a multiplexer 30 multiplexes them into a wavelength group and outputs it. The AWG 10 uses cyclically wavelength-separable elements, such as array waveguide gratings and hence is capable of wavelength conversion for a wavelength group composed of different wavelengths, thus eliminating the collision between wavelength groups of the same wavelength, even when a wavelength group exchange exchanges for each wavelength group.