A variable optical filter comprising: a first polarizer; a first phase difference element operable to generate a phase difference; a variable Faraday rotator operable to impart a variable Faraday rotation; a second phase difference element operable to generate a phase difference; and a second polarizer, wherein said polarizers, said variable Faraday rotator, and said phase difference elements are arranged in the aforementioned order along the optical axis, wherein when the rotation direction of a rotation angle θ of said Faraday rotator is positive, an angle δ formed by the transmitted polarization azimuths of the first and second polarizers is set to δ>0, the angle φ 1 formed by the optic axis of the first phase difference element and the transmitted polarization azimuth of the first polarizer is set to φ 1 <0, and the angle φ 1 formed by the optic axis of the second phase difference element and the transmitted polarization azimuth of the first polarizer is set to φ 2 <0, wherein the angles φ 1 and φ 2 are set to φ 1 ≠φ 2 and φ 1 -φ 2 ≠90°, wherein when the phase differences generated by the first and second phase difference elements (12,16) are assumed to be Δ 1 and Δ 2 respectively, at least one of the phase differences is (2n+1)λ/4 within a transmission band given in advance where λ is an arbitrary wavelength within the given transmission band; and characterised in that when the center wavelength in the given transmission band is assumed to be λ 0, the phase difference Δ 1 of the first phase difference element is 13/4λ 0 ≤Δ 1 ≤49/4λ 0, the phase difference Δ 2 of the second phase difference element is 13/4λ 0 ≤Δ 2 ≤49/4λ 0, and the variations in the mean insertion loss are 0.5dB or less at the transmission bandwidth of 40nm and over the wavelength slope from -5dB to +5dB. a first polarizer; a first phase difference element operable to generate a phase difference; a variable Faraday rotator operable to impart a variable Faraday rotation; a second phase difference element operable to generate a phase difference; and a second polarizer, wherein said polarizers, said variable Faraday rotator, and said phase difference elements are arranged in the aforementioned order along the optical axis, wherein when the rotation direction of a rotation angle θ of said Faraday rotator is positive, an angle δ formed by the transmitted polarization azimuths of the first and second polarizers is set to δ>0, the angle φ 1 formed by the optic axis of the first phase difference element and the transmitted polarization azimuth of the first polarizer is set to φ 1 <0, and the angle φ 1 formed by the optic axis of the second phase difference element and the transmitted polarization azimuth of the first polarizer is set to φ 2 <0, wherein the angles φ 1 and φ 2 are set to φ 1 ≠φ 2 and φ 1 -φ 2 ≠90°, wherein when the phase differences generated by the first and second phase difference elements (12,16) are assumed to be Δ 1 and Δ 2 respectively, at least one of the phase differences is (2n+1)λ/4 within a transmission band given in advance where λ is an arbitrary wavelength within the given transmission band; and characterised in that when the center wavelength in the given transmission band is assumed to be λ 0, the phase difference Δ 1 of the first phase difference element is 13/4λ 0 ≤Δ 1 ≤49/4λ 0, the phase difference Δ 2 of the second phase difference element is 13/4λ 0 ≤Δ 2 ≤49/4λ 0, and the variations in the mean insertion loss are 0.5dB or less at the transmission bandwidth of 40nm and over the wavelength slope from -5dB to +5dB.