A base member obtained by cutting a cylindrical body having a center axial line at a maximum asymmetric angle &agr;0 with respect to a plane orthogonal to the center axial line of cylindrical body is prepared. Next, the thus obtained ellipsoidal asymmetric cut surface of the base member is shaped along a peripheral surface of an imaginary cylindrical body having a radius R0, into an asymmetric cut curved-surface. Then, a monochromator Si crystal is bonded to the asymmetric cut curved-surface of the base member. Both the asymmetric angle and the radius of curvature for a desired wavelength within a wide wavelength range can be simultaneously tuned only by the &phgr;-axis rotation.