The invention provides a low-cost and easy-to-align optical signal processing device on which a plurality of WSS function units can be integrated using only a single lens. Optical signals input to the first input/output port group (101-1) are output into a space as collimated light via the microlens array (102). The signal light propagating through the space will be wavelength-demultiplexed by the diffraction grating (103), focused by the lens (104), and focused at the upper part in the drawing, with respect to the y-axis direction of the spatial light modulator (105). The light provided with a desired phase modulation and reflected by the spatial light modulator (105) is deflected at a desired angle in the y-z plane according to its phase setting, and further optically coupled to an arbitrary port by passing through the lens (104) again, thus the switching operation is completed.