The directional coupler switch is configured by a parallel waveguide including a first region which is subjected to a nonlinear action and a second region which is not subjected to a nonlinear action. A dispersion relationship of the first region is varied due to the nonlinear action to switch over an exit of light entering from an input side to cause the switch to function as an optical switch. A dispersion curve of the first region has: a region of a constant frequency in one of an even mode and an odd mode which are two kinds of eigen modes of the parallel waveguide; and a region in which gradients of the two eigen modes at frequencies belonging to a region other than the region of a constant frequency are substantially equal to each other. The gradients of the two kinds of eigen modes indicate monotone decreasing or monotone increasing.