An instantaneous phase-shift interferometer splitting a detected light beam into a plurality of light beams or regions, the detected light beam including a reference beam ( l 2) as a measurement reference and a measurement beam ( l 1) obtained by one of reflecting from and passing through a measured object, relatively shifting a phase difference between the reference beam ( l 2) and the measurement beam ( l 1), then generating interference fringes and simultaneously capturing a plurality of phase-shifted interference fringe images, and measuring a shape of the measured object, the interferometer comprising: a light source having a coherence length shorter than a difference in optical path length between the light reflected from a reference surface and the light reflected from a measured surface; characterized by at least one beam splitter (201; 202) configured to split the beam from the light source into two components; and a delay optical path having a changeable optical path length and configured to delay one beam of said two components to cause a difference in optical path length, wherein the beam splitter is configured to superimpose the one beam on the same optical axis as the other beam of said two components to irradiate a reference surface and a surface of the measured object, after which the reference beam ( l 2) and the measurement beam ( l 1) are generated, wherein: the instantaneous phase-shift interferometer is configured such that, in use, the optical path length of the delay optical path is changed during adjustment of the delay optical path, a plurality of phase-shifted interference fringe images are captured, and at least one of a bias, amplitude, and phase shift amount of the interference fringes obtained in each of the interference fringe images is calculated, and during measurement, the shape of the measured object is measured based on at least one of bias calculation results, amplitude calculation results, and phase shift amount calculation results of the interference fringes. a light source having a coherence length shorter than a difference in optical path length between the light reflected from a reference surface and the light reflected from a measured surface; characterized by at least one beam splitter (201; 202) configured to split the beam from the light source into two components; and a delay optical path having a changeable optical path length and configured to delay one beam of said two components to cause a difference in optical path length, wherein the beam splitter is configured to superimpose the one beam on the same optical axis as the other beam of said two components to irradiate a reference surface and a surface of the measured object, after which the reference beam ( l 2) and the measurement beam ( l 1) are generated, wherein: the instantaneous phase-shift interferometer is configured such that, in use, the optical path length of the delay optical path is changed during adjustment of the delay optical path, a plurality of phase-shifted interference fringe images are captured, and at least one of a bias, amplitude, and phase shift amount of the interference fringes obtained in each of the interference fringe images is calculated, and during measurement, the shape of the measured object is measured based on at least one of bias calculation results, amplitude calculation results, and phase shift amount calculation results of the interference fringes. the instantaneous phase-shift interferometer is configured such that, in use, the optical path length of the delay optical path is changed during adjustment of the delay optical path, a plurality of phase-shifted interference fringe images are captured, and at least one of a bias, amplitude, and phase shift amount of the interference fringes obtained in each of the interference fringe images is calculated, and during measurement, the shape of the measured object is measured based on at least one of bias calculation results, amplitude calculation results, and phase shift amount calculation results of the interference fringes.