There is provided a directional coupler including a first optical waveguide core and a second optical waveguide core that are arranged in separate and parallel to each other and that propagate one of TE polarized waves and TM polarized waves of an m-th order and propagate the other of the TE polarized waves and the TM polarized waves of an n-th order. A separation distance between the first optical waveguide core and the second optical waveguide core and a width of the first optical waveguide core and the second optical waveguide core are set such that a mode coupling coefficient of a p-th mode of one of the polarized waves and a mode coupling coefficient of a q-th mode of the other polarized waves match between the first optical waveguide core and the second optical waveguide core.