To enable evaluation of a surface roughness in an arbitrary direction within a disc surface with a magnetic recording medium using a texture substrate without being affected by a matter attracted on the surface, by measuring the surface roughness using a rocking curve method, one of X-ray diffraction methods.

A rocking curve is measured at an arbitrary location of a disc not only in a circumferential direction or in a radial direction but also in an arbitrary direction within a plane. Firstly the center of a magnetic recording medium is matched with the intersection of the Y axis and X axis. A point M0 (r, ψ) is initially moved on the Y axis to make a measuring point M. Then, an angle of deviation α is set as made by a projection line onto an XY plane of incident X rays and the X axis. The measurement of the rocking curve in the arbitrary direction within the plane, namely, the evaluation of the surface roughness is made possible by varying the deviation angle α. The use of X rays enables evaluation of the surface roughness in the arbitrary direction within the plane of the disc with a magnetic recording medium using a texture substrate without being affected by a matter attracted on the surface.