A wavelength beam combining device (100, 200, 300, 400) for combining a plurality of laser beams of respectively different peak wavelengths (λ1, λ2, λ3), the wavelength beam combining device comprising: a polarization beam splitter configured to separate the plurality of laser beams into a plurality of first polarized light beams that are linearly polarized in a first polarization direction (y axis direction) and a plurality of second polarized light beams that are linearly polarized in a second polarization direction (x axis direction) that is orthogonal to the first polarization direction; a first polarization conversion element configured to convert the plurality of second polarized light beams into a plurality of third polarized light beams that are linearly polarized in the first polarization direction; at least one diffraction grating configured to diffract the plurality of first polarized light beams and generate a first wavelength-combined beam into which the plurality of first polarized light beams are coaxially combined, and to diffract the plurality of third polarized light beams and generate a second wavelength-combined beam into which the plurality of third polarized light beams have been coaxially combined; a second polarization conversion element configured to change a polarization state of the first wavelength-combined beam or the second wavelength-combined beam so that the polarization directions of the first wavelength-combined beam and the second wavelength-combined beam become orthogonal to each other; and a polarization beam combiner configured to generate and emit a third wavelength-combined beam into which the first wavelength-combined beam and the second wavelength-combined beam have been coaxially combined. a polarization beam splitter configured to separate the plurality of laser beams into a plurality of first polarized light beams that are linearly polarized in a first polarization direction (y axis direction) and a plurality of second polarized light beams that are linearly polarized in a second polarization direction (x axis direction) that is orthogonal to the first polarization direction; a first polarization conversion element configured to convert the plurality of second polarized light beams into a plurality of third polarized light beams that are linearly polarized in the first polarization direction; at least one diffraction grating configured to diffract the plurality of first polarized light beams and generate a first wavelength-combined beam into which the plurality of first polarized light beams are coaxially combined, and to diffract the plurality of third polarized light beams and generate a second wavelength-combined beam into which the plurality of third polarized light beams have been coaxially combined; a second polarization conversion element configured to change a polarization state of the first wavelength-combined beam or the second wavelength-combined beam so that the polarization directions of the first wavelength-combined beam and the second wavelength-combined beam become orthogonal to each other; and a polarization beam combiner configured to generate and emit a third wavelength-combined beam into which the first wavelength-combined beam and the second wavelength-combined beam have been coaxially combined.