Disclosed herein is a point-diffraction interferometer which can inspect a surface quality of an optical system for extreme ultraviolet lithography using a high-order harmonic X-ray source with excellent coherence, and an apparatus and method for generating a high-order harmonic X-ray. The present invention uses a high-order harmonic X-ray beam as a coherence light source, thus remarkably reducing the size of an apparatus for generating a light source to approximately {fraction (1/100)} of a device using a light source generated in a conventional synchrotron. Further, the present invention simplifies the construction of an interferometer by employing a thin foil in which a pinhole is formed through a drilling technique using high power femtosecond laser, thus increasing the industrial utility of the interferometer.