Wavefront phase-measuring interferometers yield inevitable systematic measurement errors caused by the fabrication imperfection of constituting optical components and also by the alignment inaccuracy of assembly work. However, not like random errors, the systematic measurement errors holds some invariant rules upon the rotation of test workpieces around the optical axis, so that accurate self-calibration becomes possible without relying upon costly precision reference masters. In this dissertation a new self-calibration algorithm of multiple-step rotation is proposed with an aim to improving calibration accuracy by removing practical calibration errors induced by the inaccuracy of workpiece rotation. It is named the arbitrary-step rotation (AR) algorithm and may be regarded as an extended version of self-calibration. The rotation angles are taken as additional unknowns together with wavefront phases in the AR algorithm, and they are determined simultaneously through least squares technique. This new algorithm allows not only exact extraction of asymmetric systematic errors but also precise calibration of non-orthogonality of wavefronts in case of lateral shearing interferometers. Experimental results are discussed together with some other attempts to eliminate symmetric wavefront errors.