A planar lightwave circuit including an arrayed waveguide grating filter which has a core and a clad formed on a substrate, characterized in: that the arrayed waveguide grating filter comprises: at least one input optical waveguide adapted to inputs signal light; a first optical slab waveguide which is optically connected with said input optical waveguide; arrayed optical waveguides which are optically connected with said first optical slab waveguide, and which become longer with a predetermined waveguide length difference in succession; a second optical slab waveguide which is optically connected to said arrayed optical waveguides; and at least one output optical waveguide which is optically connected to said second optical slab waveguide; and that each of said arrayed optical waveguides comprises: mode re-coupling means for re-coupling a higher-order mode and a radiation mode to the signal light, at a part optically touching said first optical slab waveguide; and mode coupling means for coupling the signal light to the higher-order mode and the radiation mode, at a part optically touching said second optical slab waveguide; wherein 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 are configured such that a phase difference between forward propagation field and backward propagation field of the signal light at the each position is equal to or less than a predetermined error. at least one input optical waveguide adapted to inputs signal light; a first optical slab waveguide which is optically connected with said input optical waveguide; arrayed optical waveguides which are optically connected with said first optical slab waveguide, and which become longer with a predetermined waveguide length difference in succession; a second optical slab waveguide which is optically connected to said arrayed optical waveguides; and at least one output optical waveguide which is optically connected to said second optical slab waveguide; and that each of said arrayed optical waveguides comprises: mode re-coupling means for re-coupling a higher-order mode and a radiation mode to the signal light, at a part optically touching said first optical slab waveguide; and mode coupling means for coupling the signal light to the higher-order mode and the radiation mode, at a part optically touching said second optical slab waveguide; wherein 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 are configured such that a phase difference between forward propagation field and backward propagation field of the signal light at the each position is equal to or less than a predetermined error. mode re-coupling means for re-coupling a higher-order mode and a radiation mode to the signal light, at a part optically touching said first optical slab waveguide; and mode coupling means for coupling the signal light to the higher-order mode and the radiation mode, at a part optically touching said second optical slab waveguide; wherein 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 are configured such that a phase difference between forward propagation field and backward propagation field of the signal light at the each position is equal to or less than a predetermined error.