Abstract: An optical isolator comprises an optical element assembly (10) including a Faraday rotator (1) and a pair of polarizing elements (2) one of which is arranged on a light-input side of the Faraday rotator (1) to serve as a polarizer and other of which is arranged on a light-output side to serve as an analyzer. The optical element assembly (10) is received at the center of an inner bore or a cavity of a cylindrical iron-chromium-cobalt (Fe—Cr—Co) magnet (5). The Faraday rotator (1) comprises a magnetic garnet thick film which is a Bi garnet thick film remarkably low in saturation magnetization as compared with a rare-earth garnet. The Fe—Cr—Co magnet (5) serves as a field-application magnet for generating a magnetic field applied to magnetize the Bi garnet thick film and also as an isolator casing because it is excellent in magnetic and mechanical characteristics.