A measurement program is created for measurement performed by moving X- and Z-axes so that a central axis of a probe is perpendicular to the surface of a reference sphere, and errors are obtained between original probe position data and probe position data obtained by measurement performed at two different angles θ1 and θ2 of a rotary axis according to the created measurement program. Position coordinates of a tip end of the probe at the two different angles θ1 and θ2 of the rotary axis are corrected so that the errors are zero. Then, the X- and Z-axis coordinates are corrected based on a positive or negative phased shift amount, and measurement errors are obtained by calculation. A real probe tip position is defined by the X- and Z-axis coordinates corrected by a correction amount with which the obtained measurement errors become minimum.