To measure a shape of a surface to be inspected with high precision by obtaining quantitative errors of fringe scanning and feeding back the errors to the fringe scanning mechanism.

A plurality of sets of interference fringe intensity data obtained before and after step change of fringe scanning are obtained (S2). Based on the data, a linear error is calculated when the linear error is assumed to be included in variation of phase difference between an object light of the interference light and a reference light (S3), and the amount of the step change of the phase difference of the fringe scanning is corrected corresponding to the linear error (S4). When the linear error exceeds the tolerance, the fringe scanning is executed again based on the amount of the step change after the correction (S5→S2), and the initial phase distribution is calculated based on the interference fringe intensity data obtained by the re-execution (S6).

COPYRIGHT: (C)2006,JPO&NCIPI