PURPOSE: To provide an optical isolator having a constitution in which surface reflected light on a polarizer side does not return to a light source, in which area of the cylinder hole of a cylindrical permanent magnet can be used as the cross-sectional area of an effective optical path as it is, and which can be easily manufactured, mounted and assembled.

CONSTITUTION: This isolator is constituted of the cylindrical permanent magnet 1 whose both end faces orthogonally cross the center axis of the cylinder hole, a Faraday effect element board 2 fitted inside the cylinder hole of the permanent magnet 1 and arranged orthogonally crossing the center axis, a double refraction tapered board formed in a plane, whose front surface is not parallel with the back surface, and joined to the one end face of the permanent magnet 1, and a polarizing plate 8 joined to the other end face of the permanent magnet 1.