An optical isolator not dependent on the plane of polarization of an incident beam and does not allow the plane of polarization to change on the outgoing side. The optical isolator comprises a first parallel flat double refractive crystal substance; a first Faraday rotor for rotating the plane of polarization of a beam; a second parallel flat double refractive crystal substance; a second Faraday rotor for rotating the plane of polarization in a direction opposite to that in which said first Faraday rotor rotates the plane of polarization; a third parallel flat double refractive substance; a fourth parallel flat double refractive substance; and permanent magnets for magnetizing the Faraday rotor and second Faraday rotors. Since a pair of Faraday rotors for rotating the plane of polarization in opposite directions, the plane of polarization on the incident side does not change on the outgoing side.