PURPOSE: To simplify the constitution of a rotating element greatly and to reduce its size and weight by drawing a twisted part of an optical fiber linearly for looping and controlling the rotation of a plane of polarization.

CONSTITUTION: A point O of the single-mode optical fiber is fixed and a point B is twisted by one turn to form one loop, so that the point B rotates by 360° from the plane of polarization in the twisting direction. On the other hand, when the point O is fixed and the part between the points O and B is twisted by one turn, the plane of polarization of propagated light rotates by about 25° at the point B. Consequently, when light is propagated from the point O to the point B, the fiber is looped and drawn and the point B is shifted in position properly to adjust the angle of the plane of polarization at the point B by 0W35°. Consequently, the fiber part between the points O and B is usable as a polarization plane rotating element and the constitution of the element can be simplified greatly, so that the element is reduced in size and weight.