A polarizing monochromator comprising a uniaxial birefringent crystal prism, the prism has the geometry of a triangular block having a triangular base, a face including a side of the hypotenuse is an input-output face, where light enters and exits, and a face including the longer side of the right angle is a reflection face, the optic axis of the prism is perpendicular to the base of the prism, the angle of the input-output face of the prism with respect to light coming from a collimator optical system is determined in such a manner that ordinary light and extraordinary light exit from the input-output face of the prism in opposite directions with respect to an optical axis connecting the collimator optical system and the prism, and a light-collecting optical system is disposed to collect either extraordinary light or ordinary light exiting from the input-output face of the prism.