Disclosed is an interferometer that enables high-speed and high-precision measurement of a surface shape of an article and a method of producing such interferometer. Also disclosed is a method of measuring a surface shape of an article by use of such interferometer. The interferometer includes an optical system having an optical element being effective to make, into an aspherical wave, a wavefront of light to be projected on the article to be inspected, and also being arranged to be replaceable by another optical element.