To provide an optical signal processing unit that can provide a stable time order to modulation phases of optical signals in a WDM(wavelength division multiplex) light.

The optical signal processing unit of this invention is provided with an optical demultiplexer 2 and an optical multiplexer 4 for the adaptation to the WDM(wavelength division multiplex). The optical demultiplexer has an input port and output ports, and the input port 2 receives the WDM signal light that is obtained by applying wavelength division multiplex processing to optical signals having different wavelength. The optical multiplexer has an output port and input ports. Optical paths 6 interconnect the output ports of the optical demultiplexer and the input ports of the optical multiplexer. Each optical path is provided with a delay adjustment unit 8. A detector 12 detects at least one phase of the optical signals and a controller 14 controls the delay adjustment units 8 based on the detected modulation phase. Thus, the phases of the optical signals can easily be matched with each other.