An optical device comprising: a plurality of light sources (111 to 113) configured to respectively emit light of a different wavelength; a stacked wave plate that is configured by a plurality of wave plates (131 to 133) disposed in series, transmits the light emitted by the plurality of light sources (111 to 113), and induces a phase difference on polarization components of the transmitted light; a polarization adjusting unit (121 to 123)configured to adjust separately for each light source (111 to 113) of the plurality of light sources (111 to 113) a polarization direction of the light emitted by the corresponding light source (111 to 113) and input to the stacked wave plate, respectively; wherein the plurality of wave plates (131 to 133) includes: a first wave plate for which an ordinary axis is set in a given direction, a second wave plate that is disposed downstream to the first wave plate and for which the ordinary axis is set in a direction different from the given direction, and a third wave plate that is disposed downstream to the second wave plate and for which the ordinary axis is set in the given direction. and wherein the plurality of light sources (111 to 113) includes: a light source configured to emit light of a wavelength λ1, and a light source configured to emit light of a wavelength λ2 that is different from the wavelength λ1, the first wave plate is configured to have a thickness d1 and a refractive index Δn1(λ) that changes according to the wavelength λ of the transmitted light, the second wave plate is configured to have a thickness d2 and a refractive index Δn2(λ) that changes according to the wavelength λ of the transmitted light, the polarization adjusting unit adjusts the polarization directions of the light of the wavelength λ1 and the light of the wavelength λ2 such that an angle Ψ(λ1, λ2) between the polarization directions of the light of the wavelength λ1 and the light of the wavelength λ2 satisfies: Ψ λ 1, λ 2 = arcsin sin π ⋅ Δ n 2 λ 1 ⋅ d 2 / λ 1 sin 2 Ψ sin Γ e λ 1 / 2 × 1 2 − arcsin sin π ⋅ Δ n 2 λ 2 ⋅ d 2 / λ 2 sin 2 Ψ sin Γ e λ 2 / 2 × 1 2 Γ e λ = arccos [ cos 2 Ψ cos 2 π ⋅ Δn 1 λ ⋅ d 1 λ + π ⋅ Δn 2 λ ⋅ d 2 / λ + sin 2 Ψ cos 2 π ⋅ Δn 1 λ ⋅ d 1 / λ−π ⋅ Δn 2 λ ⋅ d 2 / λ × 2 where, an angle between the ordinary axes of the first wave plate and the second wave plate is Ψ. a plurality of light sources (111 to 113) configured to respectively emit light of a different wavelength; a stacked wave plate that is configured by a plurality of wave plates (131 to 133) disposed in series, transmits the light emitted by the plurality of light sources (111 to 113), and induces a phase difference on polarization components of the transmitted light; a polarization adjusting unit (121 to 123)configured to adjust separately for each light source (111 to 113) of the plurality of light sources (111 to 113) a polarization direction of the light emitted by the corresponding light source (111 to 113) and input to the stacked wave plate, respectively; wherein the plurality of wave plates (131 to 133) includes: a first wave plate for which an ordinary axis is set in a given direction, a second wave plate that is disposed downstream to the first wave plate and for which the ordinary axis is set in a direction different from the given direction, and a third wave plate that is disposed downstream to the second wave plate and for which the ordinary axis is set in the given direction. and wherein the plurality of light sources (111 to 113) includes: a light source configured to emit light of a wavelength λ1, and a light source configured to emit light of a wavelength λ2 that is different from the wavelength λ1, a first wave plate for which an ordinary axis is set in a given direction, a second wave plate that is disposed downstream to the first wave plate and for which the ordinary axis is set in a direction different from the given direction, and a third wave plate that is disposed downstream to the second wave plate and for which the ordinary axis is set in the given direction. and wherein the plurality of light sources (111 to 113) includes: a light source configured to emit light of a wavelength λ1, and a light source configured to emit light of a wavelength λ2 that is different from the wavelength λ1, a light source configured to emit light of a wavelength λ1, and a light source configured to emit light of a wavelength λ2 that is different from the wavelength λ1, the first wave plate is configured to have a thickness d1 and a refractive index Δn1(λ) that changes according to the wavelength λ of the transmitted light, the second wave plate is configured to have a thickness d2 and a refractive index Δn2(λ) that changes according to the wavelength λ of the transmitted light, the polarization adjusting unit adjusts the polarization directions of the light of the wavelength λ1 and the light of the wavelength λ2 such that an angle Ψ(λ1, λ2) between the polarization directions of the light of the wavelength λ1 and the light of the wavelength λ2 satisfies: Ψ λ 1, λ 2 = arcsin sin π ⋅ Δ n 2 λ 1 ⋅ d 2 / λ 1 sin 2 Ψ sin Γ e λ 1 / 2 × 1 2 − arcsin sin π ⋅ Δ n 2 λ 2 ⋅ d 2 / λ 2 sin 2 Ψ sin Γ e λ 2 / 2 × 1 2 Γ e λ = arccos [ cos 2 Ψ cos 2 π ⋅ Δn 1 λ ⋅ d 1 λ + π ⋅ Δn 2 λ ⋅ d 2 / λ + sin 2 Ψ cos 2 π ⋅ Δn 1 λ ⋅ d 1 / λ−π ⋅ Δn 2 λ ⋅ d 2 / λ × 2 where, an angle between the ordinary axes of the first wave plate and the second wave plate is Ψ.