Of two pairs of biaxial goniometers and a uniaxial straight-ahead stage, one pair of biaxial goniometers and the uniaxial straight-ahead stage are subjected to fully-closed feedback control (follow-up control) under which output from a QPD is directly input into an axis drive motor, and the remaining pair of biaxial goniometers are subjected to semi-closed feedback control (constant-value control), encoder outputs on all the axes and QPD output are acquired simultaneously, measurement point coordinates and normal vectors derived from the encoder outputs are corrected with the QPD output, thereby eliminating influence of steady-state deviation in a goniometer control system.