PURPOSE: To reduce the size and cost of a device by combining one polarizer with a magnetooptic element which has a slanting surface cut at the complementary angle of a Brewster angle on one end surface and performs polarizing operation on the slanting surface.

CONSTITUTION: Light L12 reflected by an end surface of an optical fiber 5 passes through the polarizer 11 in the opposite direction shown by B and is made incident to the vertical surface 13 of the magnetooptic element 10. The light is influenced therein by an external magnetic field H and its plane of polarization rotates counterclockwise about an optical axis. Linear polarized light reaching the slanting surface 12 while the plane of polarization rotates by 45° has the plane of polarization rotated by 90° as compared with light A traveling forward. Consequently, the light is reflected by the slanting surface 12 which is cut at the complementary angle of the Brewster angle and provides polarization, obtaining reflected light L13. Therefore, almost no light returns to a semiconductor laser 4. Thus, the size and weight of the optical isolator are reduced.