Abstract: This invention relates to a fiber optic polarization controller which controls the polarization direction of the light propagating through an optical fiber by curving a single-mode fiber within an imaginary plane S to generate birefringence on the fiber and by rotating the plane S defined with the thus curved portion. The invention aims to prevent twists of the optical fiber which are caused in rotating the plane S by rotating the fiber in one direction from the plane S simultaneously with the rotation of the plane S and correspondingly to the angle thereof. The optical fiber may be rotated either by utilizing the stability of fiber which is held loosely at a curved portion or by forcibly rotating the curved portion from outside.