A planar lightwave circuit including an optical branch circuit which has a core and a clad formed on a substrate, characterized in: that the optical branch circuit comprises: one input optical waveguide which inputs signal light; mode coupling means for coupling part of the inputted signal light to a higher-order mode and a radiation mode; mode re-coupling means for re-coupling the signal light coupled to the higher-order mode and the radiation mode by said mode coupling means, to output signal light; and at least two output optical waveguides (315a, 315b) which output the output signal light; said mode coupling means and said mode re-coupling means being optical waveguides each of which has a core width varied continuously, wherein refractive-index-distribution of said optical waveguides at boundaries between core and clads at each position (x, z) along propagation direction (z) of the signal light in said mode coupling means and said mode re-coupling means is configured such that a phase difference between forward propagation field and backward propagation field of the signal light at the each position becomes equal to or less than a predetermined error, wherein the variation range of the core width of the optical waveguide is set at ±4.0 μm or ±8.0 μm per unit length in the propagation direction of the signal light, wherein the minimum value of the optical-waveguide distance between adjacent optical waveguides among the optical waveguides each of which has a core width varied is set at 1.0 μm, and wherein the branching angle between the adjacent optical waveguides is set at 2.5 degrees. that the optical branch circuit comprises: one input optical waveguide which inputs signal light; mode coupling means for coupling part of the inputted signal light to a higher-order mode and a radiation mode; mode re-coupling means for re-coupling the signal light coupled to the higher-order mode and the radiation mode by said mode coupling means, to output signal light; and at least two output optical waveguides (315a, 315b) which output the output signal light; one input optical waveguide which inputs signal light; mode coupling means for coupling part of the inputted signal light to a higher-order mode and a radiation mode; mode re-coupling means for re-coupling the signal light coupled to the higher-order mode and the radiation mode by said mode coupling means, to output signal light; and at least two output optical waveguides (315a, 315b) which output the output signal light; said mode coupling means and said mode re-coupling means being optical waveguides each of which has a core width varied continuously, wherein refractive-index-distribution of said optical waveguides at boundaries between core and clads at each position (x, z) along propagation direction (z) of the signal light in said mode coupling means and said mode re-coupling means is configured such that a phase difference between forward propagation field and backward propagation field of the signal light at the each position becomes equal to or less than a predetermined error, wherein the variation range of the core width of the optical waveguide is set at ±4.0 μm or ±8.0 μm per unit length in the propagation direction of the signal light, wherein the minimum value of the optical-waveguide distance between adjacent optical waveguides among the optical waveguides each of which has a core width varied is set at 1.0 μm, and wherein the branching angle between the adjacent optical waveguides is set at 2.5 degrees.