An optical isolator having an optical element, comprised of an incoming side polarizer, a Faraday rotator, and an outgoing side polarizer superposed in the order of mention in the direction of an optical path, is embedded in the optical path of an optical fiber or optical waveguide having a light axis. The isolator has a slot extending across the light axis in a direction slanted with respect to the light axis. The optical element is inserted in the slot and the spaces between the optical element and the incoming and outgoing sides of the slot are filled with an optical adhesive portion. At least one of the interfaces is inclined at an angle reversely from the plane normal to the light axis as viewed from the slanted incoming side of the slot.