To obtain information as an index of how correctly a fringe scan is conducted by constituting an interferometer apparatus using the fringe scan to carry out a predetermined analysis with utilization of interference fringe image data obtained when interference fringes are measured.

Interference fringe image data is taken every time a reference face 22a moves by a predetermined step amount because of a fringe scan, and an interference fringe intensity at each point of a face 2a to be detected is measured. With the use of the measurement result, a phase, an average light intensity and a modulation amount of each point of the face 2a are calculated. An interference fringe intensity of each point of the face 2a at each step of the fringe scan is calculated from the calculation result by an inverse operation. An error in phase change at each step is obtained by calculating a difference between the inversely operated interference fringe intensity and the interference fringe intensity obtained by the measurement.