To provide a lithium niobate (LN) crystal polarizing element with high production yield and to provide an optical isolator at a low cost by using the same element.

The polarizing element for an optical isolator is a birefringent wedge type light polarizing element having an inclined face, and the polarizing element is made of a lithium niobate crystal. The optical axis of the crystal is inclined by ±67.5° from the direction of the lines of equal thickness in the wedge of the polarizing element. An isolator unit is prepared by arranging two of the above birefringent type polarizing elements in such a manner that the direction of the lines of equal thickness in the wedges is identical, that the optical axis of the one polarizing element is inclined by +67.5° from the direction of the lines of equal thickness and that the optical axis of the other polarizing element is inclined by -67.5° from the line of equal thickness. A two-stage optical isolator is obtained by connecting two of the above optical isolator units in such a manner that the inclined faces of the two polarizing elements opposing to each other are shifted by 90° from each other and that the optical axes of the two elements are shifted by 90° from each other.