A laser tracking interferometer; a carriage rotatable about a center of the reference sphere; a laser interferometer that provides a retroreflector displacement signal, and a displacement gage that provides a displacement signal. A data processing apparatus computes displacement of the retroreflector with respect to the reference sphere. A position sensitive detector provides a position signal corresponding to the amount of deviation of a laser beam when the laser beam is reflected off the retroreflector back into the laser interferometer and deviated in a direction orthogonal to its optical axis. A controller controls rotation of the carriage based on the position signal from the position detector so that the amount of deviation becomes zero.