In conventional optical isolators, an optical signal is dispersed by polarization or the characteristics are varied by heat generation in a garnet crystal. According to the invention, the crystal optical axis (3c) of rutile crystal (3) is oriented so that the separation directions of the ordinary ray (O) and the extraordinary ray (E) are perpendicular to the plane including the optical axes of optical fibers (10, 11). Furthermore, the focusing central optical axis (6c) of a focusing rod lens (6) is arranged parallel with optical axes of the optical fibers (10, 11) and at a substantially equal distance from the four rays, i.e. the ordinary ray (O) and the extraordinary ray (E) propagating along the optical axis of the optical fiber (10) and the ordinary ray (O) and the extraordinary ray (E) propagating along the optical axis of the optical fiber (11). An air gap (7) of about 200 [.mu.m] is provided, as a heat insulating means, between the focusing rod lens (6) and a magnetized garnet crystal (8).