A heat-dissipating structure for an optical isolator is capable of suppressing an increase in temperature caused by light absorption in a magnetic garnet crystalline film by radiation fins extending from the inside of an external heat conducting cover. The heat-dissipating structure for the optical isolator is formed by housing a magnetic garnet crystalline film (12), first and second heat conductive plates (6, 7, 8 and 9) and magnet 18 in the external heat conducting cover, placing the radiation fins (10 and 11) on the second heat conductive plate, attaching the first heat conductive plates (6 and 7) onto either side of the magnetic garnet crystalline film, arranging the second heat conductive plates (8 and 9) on the outer surface of the first heat conductive plates, and passing the radiation fins through guide openings (2a and 2b) in the isolator holder 2 to the outside of the external heat conductive cover from the extracting opening (3c) to be brought into contact with the outer grooves (4d and 5d).