To provide an optical signal processing device capable of reducing the crosstalk while narrowing the space between switch elements for downsizing, the optical signal processing device includes a plurality of input optical waveguides, a plurality of output optical waveguides, a plurality of optical waveguide elements arranged between the plurality of input optical waveguides and the plurality of output optical waveguides, and a connection optical waveguide. The connection optical waveguide positioned closely to the optical waveguide element is differentiated in propagation constant from the optical waveguide configuring the closely arranged optical waveguide element. The connection optical waveguide positioned closely to the optical waveguide element is a connection optical waveguide having one end or both ends connected to the optical waveguide element, or a connection optical waveguide having both ends not connected to the optical waveguide elements.