To provide a light wavelength separation device and a light wavelength separation method that can be flexibly adapted for various channel intervals of a wavelength-division multiplexed (WDM) signal, a light wavelength separation circuit is provided with: an optical coupler which splits a wavelength-multiplexed optical signal in which optical signals of a plurality of channels are multiplexed; a band-pass filter which is arranged for each of output ports of the optical coupler, separates optical signals included in the wavelength-multiplexed optical signal inputted from the output ports of the optical coupler into channels of which the central frequencies are not adjacent to each other, and outputs the separated optical signals from respectively different output ports; and an optical switch which selects one of paths of the optical signals inputted from the output ports of each band-pass filter.