To provide an optical isolator which realizes simplification, miniaturization, and cost reduction of the structure of an optical isolator mounted on an amplifier, and has improved characteristic by reducing PMD and PDL to desired numerical levels.

The optical isolator is composed of: two birefringent elements; a 45 degree Faraday rotator; a magnet; and a total reflection member, and mutual directions of crystallographic axes of the two birefringent elements and a rotational direction of the Faraday rotator are suitably set, and thereby a both-way type optical path is formed which separates non-polarized light of incidence into a normal light beam and an abnormal light beam and then makes the total reflection member reflect the beams of light, and the direction of the crystallographic axis to either of the element surface normal directions of the two birefringent elements and the normal directional thickness of the element surface are corrected.