A planar optical waveguide device, includes: two input portions that are waveguides that have the same width, are parallel to each other, and have rectangular cross-sections; a wide portion that is a linear waveguide and is connected after the two input portions; a tapered portion that is connected after the wide portion and that is a multi-mode waveguide which has a tapered shape having a width decreasing gradually and through which at least TE1 propagates; and an output portion that is connected after the tapered portion and that is a multi-mode waveguide which has a rectangular cross-section and through which at least TE1 propagates. The planar optical waveguide device forms a high-order mode conversion combining element that outputs the TE0, which is input to the two input portions, as the TE1 from the output portion.