PURPOSE: To decrease the width of parallel magnetic domains, increase the angle of diffraction without increasing the thickness of magnetic material films and improve the utilization efficiency of light by forming the parallel magnetic domains within the films of magnetic material having magnetic anisotropy and providing a plurality of the films of this magnetic film in close contact or in proximity.

CONSTITUTION: The ones comprising using, e.g., gadolinium gallium garnet as substrates 29, 30 and forming magnetic material films 31, 32 having vertical anisotropy to the film plane of yttrium iron garnet added with Bi on their surfaces are disposed in close contact or by leaving a small distance D. DC or AC magnetic fields are applied horizontally and vertically to the substrates 29, 30 film surfaces to form parallel magnetic domains 16aW16c..., 17aW17c..., 18aW18c..., 19aW19c... as shown in Fig. II, respectively on the films 32, 31. Thus in order to increase the film thicknesses h1, h2 must be increased with only one magnetic film. On the other hand, in order to increase the angle of diffraction, the film thickness h1, h2 must be decreased to decrease the parallel magnetic domain width. By avoiding such contradicting conditions in this way, the polarizing device of good efficiency is obtained.