The possibilities of applying the point diffraction interferometry (PDI) method for the detection of the middle spatial frequency roughness of superpolished optical surfaces are analyzed. The point source used in the experiment is based on a single mode optical fiber with the subwavelength exit aperture size, which is about 0.25 μm. In a numerical aperture of 0.01 the reference wave root-mean-square deformation is less than 0.005 nm. It is theoretically shown that the possible diffraction-limited lateral resolution of PDI while measuring a spherical substrate of 100 mm curvature radius is about 8 μm. The experiment demonstrated the possibility of obtaining roughness spectra in the range 0.001-0.05 μm(-1). The surface map obtained by PDI, and the roughness spectra obtained by both the PDI and atomic-force microscopy methods are shown.